Archive of Our Own ^beta

Chapter Text

 Eggman was up to no good. Again.

  Then again, when wasn’t he?

If you asked Sonic, he’d tell you he almost preferred it that way. A scheming Eggman meant something to chase, something to stop, something fun. Sure, peace and quiet were nice sometimes, but Sonic was a guy who lived for motion, for the next wild ride, the next mad dash. Eggman’s nonsense kept things interesting. Honestly, it’d be kind of sad if the guy ever just… stopped.

There was always another adventure waiting, another loop in the rollercoaster. That was the way of Sonic the Hedgehog, the fastest thing alive. Always moving, always running forward.

This time, Eggman’s plan wasn’t even all that shocking. The guy had built another massive contraption, something intergalactic this round. From what Tails had picked up, it looked like Eggman was using a few Chaos Emeralds to power some kind of dimensional-hopping device, something that could jump across worlds, harvest materials, and make him even more dangerous than before.

Sonic could admire aiming big, but that was pushing it. Dragging other worlds into their mess? Nah, that was where he drew the line.

He tapped a finger to the communicator in his ear just as static cleared and Tails’ voice came through.

“Alright Sonic, Eggman’s just up ahead. Once you pass through this zone, you should see the world-hopping machine. Be careful, okay?”

Sonic grinned. “Careful? Where’s the fun in that?”

With that, he took off, boosting forward at blinding speed, wind howling around him as the sound barrier shattered like glass. Badniks exploded into scrap behind him as he chain-homing-attacked through their ranks, flipping through the air and landing in one clean motion. He hit the final chamber in seconds, dust swirling around his shoes.

“So, doc, trying to target other worlds now? I thought what we had was special.”

“Geh!? Sonic!?” Eggman spun around, eyes wide. “Tch, I knew Metal wouldn’t have been enough. Sage! Deploy the Giganto Mech!”

“Very well, Father.”

Light flashed and a familiar digital ripple surged to life beside him. Pieces of metal formed from thin air, snapping together into a miniaturized version of the Giganto mech. Sage’s holographic form flickered, then merged into it, piloting it directly.

Sonic smirked, wiping his nose with his thumb. “Heh. Give me—”

His eyes darted to the machine behind Eggman. All seven Chaos Emeralds hovered in a circle, connected by glowing cables of red energy.

“—fifty seconds tops. Time me.”

He curled into a ball, charging energy until the floor beneath him cracked, then launched himself forward with a powerful spin dash. He tore through the machine, striking the emerald array dead center. A flash erupted—brilliant gold light flooding the room, raw chaos energy surging outward in a wave that sent Eggman tumbling back.

Out of the explosion emerged a figure burning with golden energy—the incarnation of chaos itself. Super Sonic.

Sage paused, the mech’s eye-lights dimming slightly.

“Father, our chances of success are approximately zero percent.”

Eggman’s mustache twitched as he slammed his hands on his console. “Hold him off!”

Sage nodded, and the mech charged forward, but Sonic was already moving, a streak of light circling it faster than the eye could follow. Every punch Sage threw was effortlessly dodged, every missile intercepted. He wasn’t even trying.

The mech swung again, a massive metallic punch aimed straight at him. Sonic stopped it mid-swing with one hand and let out a yawn.

“Gotta do better than that.”

In the blink of an eye, he darted upward, curled into a spinning blur, and shredded through the mech’s armor piece by piece. Sparks flew. Circuits burst. The whole thing erupted in a cascade of fire and light.

He landed softly, brushing off dust that wasn’t there. “Knew she’d be fine,” he muttered, glancing at the fading data where Sage had already escaped from the wreckage. Then he turned toward Eggman.

“Alright, let’s wrap this up—whoa!”

The machine behind Eggman suddenly roared to life, sparks leaping from its surface. Sonic’s ears perked. That wasn’t supposed to happen. It shouldn’t be able to happen—the Chaos Emeralds were the power source, and he had them. Unless—

He looked down at his glowing form. His golden aura was shifting, turning into swirling blue light.

Eggman’s laughter echoed over the sound of tearing energy.

“That’s right, hedgehog! I recalibrated the device to use you as the catalyst! Now, with it activated, you, the machine, and the Emeralds will all be blasted into another dimension! There’s no escape this time, end of the line!”

“Huh!? Are you insane!? Don’t you need the Emeralds for your plan!? That’s reckless even for you, doc, hey!”

Before he could react, Sonic felt his energy rip away from him. The Chaos Emeralds detached, torn from his aura one by one as his form dimmed back to normal. A powerful gravitational pull dragged him toward the swirling vortex.

“It doesn’t matter, Sonic! With you gone, my plans will proceed uninterrupted! Goodbye!”

“Yeah right, like I’d just—” His voice was drowned out as the machine imploded.

The vortex collapsed inward, swallowing Sonic, the Emeralds, and the entire device in a blinding burst of light. The last thing he saw was Eggman’s triumphant grin before everything went white.

Everything turned white.

It was like getting smacked with a flashbang of pure chaos energy. The light burned through his vision until there was nothing, no color, no sound, no motion, just a sharp, empty hum in his head. Then, slowly, the world started to take shape again.

Grass. Soft, damp, and everywhere. Sonic groaned, feeling his body ache in places he didn’t know could ache. His eyes blinked open, squinting past the blur until the scene finally came into focus. He was lying face-first in a massive crater surrounded by flattened grass, like he’d been dropped from orbit.

“...Ugh. Guess the landing could’ve been worse.”

He pushed himself up and immediately noticed something strange. Above him stretched a night sky, but it wasn’t his sky. The stars were positioned differently. And the moon, perfectly round, whole, and unbroken. No cracks, no shattered fragments drifting around it.

“Huh. Definitely not home.”

He exhaled and rubbed the back of his head, scanning the area. No sign of the Chaos Emeralds. No sign of Eggman’s machine either. Both were gone, ripped out of existence, just like that.

“Well, that’s just great,” he muttered. “All that effort, and I don’t even get to punch Eggman one last time before being teleported to a new world for the third time.”

He stretched his arms out to shake off the stiffness, but froze halfway.

His hands.

They weren’t his.

No fur. No gloves molded perfectly for hedgehog hands. Just… human fingers. Five of them, perfectly normal, moving exactly how they shouldn’t.

“Wait, hold on—”

He looked down at himself and blinked hard, once, twice, then again. He wasn’t small anymore. His whole body had changed, longer limbs, taller frame, more… well, human-shaped. His skin was smooth instead of furred, though his arms still had faint blue markings that trailed up like stylized lightning streaks.

And his ears, oh, they were still pointed, just shaped differently, twitching slightly at every distant sound. He reached back, half afraid of what he’d find, and sure enough…

A tail.

A long, silky tail—blue, swaying gently behind him.“What in the world…”

The words came out of his mouth, but the sound wasn’t his either. It was higher. Softer. Lighter.

He paused, mouth half open, and repeated the line in disbelief. “What in the world.”

Yep. Definitely a girl’s voice.

“...Wait a second.”

Sonic—well, she now—looked down at herself again. The outfit was completely different too. A cropped white hoodie top trimmed with red and blue, showing a surprisingly toned midriff. Loose blue athletic pants with a red-and-white stripe down the side and a golden star buckle holding a ribboned belt. Red sneakers with gold clips. Her hair, still that same iconic cobalt blue, was tied up in a messy ponytail that bounced with every tiny movement.

The overall look screamed “Sonic,” but there was something undeniably… different. Her body looked lean but strong, every line made for speed. And that tail. Definitely equine. Definitely not hedgehog.

Sonic stared down for a long second, then sighed. “You’ve gotta be kidding me.”

It took a few seconds for it to really sink in. She wasn’t just in another world, she wasn’t even the same species anymore.

She—he—whatever—was some kind of… horse girl.

“Of all the possible worlds I could’ve ended up in…”

A nervous laugh slipped out as she brushed her hair back and stared up at the sky again. Her emerald-green eyes caught the moonlight, shimmering faintly like chaos energy itself.

“Great. Sonic the Horse. That’s new.”

If Amy ever saw her like this, she’d never live it down. And Tails? He’d definitely have a field day analyzing this kind of transformation. Still… it wasn’t the worst outcome, all things considered. She’d been through weirder. Being turned into a werehog once kind of set the bar high.

“Alright. Let’s see what we’re working with.”

She stood up, stretching her arms overhead before glancing down at her new legs. They were longer, faster-looking. Her balance felt weirdly perfect, like her body was designed to sprint. Curiosity sparked in her chest, the same kind that always kicked in before a race or a good challenge.

“Can I still even run fast?”

There was only one way to find out.

She took a deep breath, bent her knees, and dashed.

The wind caught her instantly, whirling around her like old friends. Blue streaks of energy sparked off her as she circled the crater, each step kicking up tiny gusts. Her movements felt different, lighter, freer, but still hers. The sound of the air rushing past her ears, the rhythm of her feet hitting the ground, it was all familiar, even if the body was new.

After a few loops, a small tornado whipped up in the center, spinning grass and dirt into the air. Sonic skidded to a stop, shoes screeching against the ground as the whirlwind faded.

She grinned, wiping nonexistent sweat from her forehead. “It’s a bit awkward, but I still got it.”

Yeah, she might not be a hedgehog anymore, but speed was still in her blood. Whatever she was now, horse girl or whatever this dimension called it, she could still run circles around anything.

Her attention turned toward the horizon. Past the edge of the crater, she could see distant lights, bright, colorful, almost festival-like. Neon signs reflected off rooftops, and somewhere in the distance, she could hear the faint echo of music and chatter.

“Huh. So that’s civilization, huh?”

From the architecture and layout, it looked distinctly modern. She squinted, making out signs in Japanese characters.

“Yep. Japan. Figures.”

She placed her hands on her hips, her ponytail flicking behind her as she took in the sight. The air was different here, calmer, cleaner. The stars shone brighter, and the faint scent of cherry blossoms drifted from somewhere nearby.

“Well, wherever this is, I can’t just stand around.”

She stretched her arms one more time and took a step forward, her sneakers crunching against the grass. The night wind brushed against her ears, making them twitch involuntarily. It was still weird, feeling things like that. But… it wasn’t bad.

“Alright. Emeralds first, explanations later.”

Her eyes lit up with that familiar spark of determination. “Can’t slack now. Let’s see what this world’s got.”

With that, Sonic, now a bright blue horse girl with wind swirling at her heels, took off toward the city lights, her silhouette cutting across the moonlit field like a streak of sapphire lightning.

She didn’t know where she was going, who she’d meet, or how she’d get home. But that was fine.

Every new world started the same way, with one step forward and a whole lot of running.

Chapter Text

Sonic wasn’t a stranger to traveling. For basically her entire life, she’d been on the move. It was just who she was. She’d run across continents, through different dimensions, and even to other worlds entirely. She never really stuck to one place, unless you count the pile of wreckage she called an island home. That was just where she threw junk and leftover adventure gadgets. Being an adventurer meant she was used to wandering, and honestly, she liked it that way.

The adventure with Chip from a while back reminded her just how fun it was to travel around the world, seeing everything for the first time like a tourist with super speed. That feeling never got old.

Because of all that, she knew her way around Japan pretty well. She’d been here before, more than once. Sonic also had this weird innate ability to understand pretty much any language, even ones she never studied. She never questioned it too hard, it just worked. That included Japanese, which she was fluent in. (How else was she supposed to talk to a certain popstar sibling if she couldn’t speak Japanese?)

So she knew immediately that she was in Fuchu, Tokyo. The geography, the stations, the layout, it all checked out. For hours she ran at super-sonic speeds, scouting everything she could. She wanted information, anything to give her a clue about where she was and what Eggman may have done this time.

At first glance, it really was just Tokyo. Crowded streets, neon signs, people everywhere. No chaos beasts roaming around, no weird gravity zone, no time distortion. Just… Japan. A version of Japan, anyway.

Except something was really different.

There were no animals like herself. No Mobians, no chao, nothing. Just humans, wall to wall. That part wasn’t strange on its own, plenty of places in her world were mostly human. The real difference was the Horse Girls.

Horse Girls. Or “Umamusume,” as everyone called them here.

They were everywhere. Posters, storefronts, billboards, Sonic couldn’t go ten seconds without seeing one. Even more surprising was how normal they were to everyone else. They weren’t some hidden secret species or legendary mythical tribe. They were just part of daily life. People treated them like athletes, celebrities, idols. There were even special running lanes built just for them, which Sonic was currently using to zip around. Honestly? Pretty convenient.

From what she could tell, they weren’t hiding from humans, they’d integrated so well that they were basically co-stars of society. She saw them running for sport, hanging out with regular people, even working normal jobs. Everyone just accepted it.

Still, even after hours of scouting, Sonic had found no Chaos Emeralds, no energy signatures, no robot stomping through the city. No sign of the machine that brought her here in the first place. Just this strange new world filled with horse girls and absolutely no clue where to start.

She knew she had to get back eventually. That was never in question. But for once, maybe running aimlessly wasn’t going to help her. Even she had to admit that.

Though, knowing her, she was probably going to run around aimlessly anyway.

Just with slightly more purpose this time.

Just as Sonic was about to take off again to see what she could find…

“WAAAAIT!”

“Huh….?”

Being a trainer was rough.

That was the thought constantly sitting in the back of Yuji’s mind. Every time she woke up, every time she walked through the gates of Tracen Academy, every time someone asked her how her training was going, it repeated like background noise: Being a trainer is rough.

Her name was Yuji, and somehow, for reasons beyond her understanding, she was a trainer for Umamusume.

She still wasn’t sure how she got here. It wasn’t like she grew up wanting this. She didn’t grow up dreaming about horses, or racing, or cheering from the sidelines. If anything, she originally wanted to be an engineer. That had been her real dream, ever since she was a kid. She wanted to invent things. Create something amazing. Leave her mark on the world with gears and blueprints and machines that would make people say “Wow, you made that?”

She loved tools, and wires, and taking things apart just to see how they worked. She made silly little gadgets as a teen, things like cheap binoculars held together by tape and hope, but she liked building things. It made her feel capable.

But reality hit harder than she expected.

No matter how much she studied or how many lessons she took, her inventions always fell short. Nothing came out the way she imagined. Her grades were average at best, and her professors always said the same thing:

“You put in effort, but it lacks polish.”

“You understand the idea, but not the execution.”

Her parents were less gentle about it.

“You’re not cut out for this, Yuji.”

“You’re wasting your life. Try something easier.”

She kept pushing anyway. She kept working. She spent years studying science, years of her life, only to watch all of it crumble when she realized she wasn’t improving fast enough. Everyone else was moving forward, getting internships, working in labs, inventing real things.

She was stuck making scrap-bin binoculars and crying over broken robotics parts.

So, eventually, she quit.

She shut down her workshop. Packed everything away in boxes she still couldn’t bring herself to throw out. Walked away before she even had a chance to get off the ground.

And somehow, through one strange recommendation after another, she ended up here, working as a trainer for Horse Girls.

She didn’t know why she said yes. Maybe she panicked. Maybe she wanted something to work out. Maybe she was just tired of disappointing everyone and thought, sure, maybe horses are easier than circuits.

(They weren’t.)

She wasn’t physically active, she didn’t have an athletic bone in her body, and she absolutely did not have the confidence or charisma trainers were supposed to have. She didn’t have bold speeches or fiery determination to inspire anyone. She didn’t even know what to say to half the girls she met.

And the results showed.

Her first few Uma Musume didn’t improve. They didn’t get faster. One quit racing entirely. Another transferred to a better trainer. Yuji apologized every time. She meant it every time. And every time, it felt worse.

Now she was one mistake away from being fired. Her supervisor made that very clear.

“If you can’t secure an Uma and start getting results soon, we’ll have to let you go.”

She was on the verge of giving up again. Maybe she really wasn’t cut out for this. Maybe she should go work in fast food, flip burgers, live in a cheap rented apartment and pretend she didn’t once have dreams.

She sat on a bench that night, eating cheap ice cream alone, staring up at the sky because looking down only made her feel like she might cry.

And that’s when she saw it.

A shooting star, bright, fast, and weirdly close. It streaked over the city like a blue comet, then something exploded in the distance. A crash, like a meteor hitting dirt.

Yuji blinked, then squinted.

“…Huh?”

She scrambled through her bag, digging out her old homemade binoculars (still taped together, still kind of terrible), and pointed them toward the crash site.

And she saw her.

A Horse Girl. Or Uma. Or something like an Uma anyway.

She had never seen one like that, though.

Her hair wasn’t normal. Her ears weren’t quite like the others. And she was fast—really fast. Even without racing, just running freely, the girl was kicking up gusts of wind strong enough to toss trash cans and rattle street signs. Every few seconds, she became a blur, blue and impossible to track.

Then, in less than a minute, she vanished completely.

Yuji lowered her binoculars slowly.

“…That… is my ticket to success.”

A fast Uma. A ridiculously fast Uma. Someone who already had speed built in. Someone she wouldn’t have to train from scratch.

If she could get that Uma and convince her to be trained… she might actually keep her job.

So, all day, she tracked her.

It was a miserable experience, honestly. Every time she tried to follow, the girl sped off again. Yuji’s legs were nowhere near fast enough to keep up. She took subways, cut corners, asked random people if they’d seen a “blue Uma with tornado hair.”

Most of them thought she was crazy.

But she kept going, because this felt like her last chance. If she failed this, then that was it. Game over.

She checked public tracks, parks, riversides—anywhere a runner might go. She even tripped at one point and scraped her knees so badly she had to stop at a convenience store for bandages.

But finally—finally—after hours of desperate tracking, the girl slowed down near a residential running lane. Yuji didn’t even think. She just ran, waving her arms.

“Heeeey! WAAAIT!”

The blue Uma turned slightly, confused.

“Wuah? …Huh?”

Up close, Yuji got her first real look, and immediately blurted out the first thing that came to mind.

“Woah, you’re much shorter in person.”

The Uma blinked at her. Yuji didn’t even know why she said it. She just panicked.

The girl really was short, though—like… 4’11. Maybe shorter. Definitely not the height she expected for someone who ran like a supersonic bullet.

Yuji stood there, breathing hard, hair messy, knees bandaged, hands shaking slightly from nerves and exhaustion.

This was it.

Her last shot at being a trainer.

She swallowed, trying to gather whatever confidence she had left.

“What’s the deal, lady? Why did you suddenly start yelling at me?”

The blue Uma skidded to a stop, one heel digging into the pavement with a sharp screech that kicked up dust. Her tail flicked behind her, ears perked in confusion.

Yuji was already doubled over trying to catch her breath. Her lungs burned, her legs felt like noodles, but she forced herself to stand up straight.

“Um…! Can I train you?!”

The Uma blinked.

“…Train… me?” She tilted her head. “Hey, listen, what’s that supposed to mean?”

Yuji froze. Oh right. She fell from the sky. She might not know anything. She might literally be an isekai protagonist in Uma Musume form.

Yuji scrambled to explain.

“Sorry! That was really sudden. My name is Yuji. I’m a trainer from Tracen Academy—well, technically I am, if I don’t get fired by the end of the week. Tracen is a place where horse girls like you are trained to race officially.”

She pushed her glasses back up her nose. They were slipping because of sweat and panic.

“I saw you fall from the sky earlier, and ever since then I’ve been trying to follow you.”

“With that thing?”

Yuji blinked, confused—until the blue Uma pointed at her hand.

The binoculars.

Her stupid, taped-together, scrap-metal binoculars she made years ago.

Oh no.

“Uh… yes. W-With this thing…”

The Uma stared at it for a long second. Then her face lit up.

“Wow! Not too shabby! If you managed to keep track of me with those things, then you must be pretty good at making stuff, Yuj!”

She said it so casually—Yuj. Not Yuji. Just decided to nickname her instantly.

Yuji’s brain completely stalled.

This Blue Uma…

…is a real weirdo.

“Th-Thank you… I think. Wait, no, we’re getting off track—” Yuji snapped back to reality. Time was precious. Her job was on the line.

“I wanted to see if you’d let me train you! I don’t even need you to do real races yet—I just need to enroll you as my Umamusume, then I can keep my job! Please!”

She bowed. Hard. Nearly 90 degrees. Her forehead almost smacked her knees.

She stayed like that, tense and sweating, waiting for rejection.

Instead, she heard:

“I don’t need all your bowing, I heard ya loud and clear.”

Yuji slowly lifted her head.

The Blue Uma had her arms crossed and was nodding, like she had already made up her mind.

“I’ll tag along with you.”

Yuji’s heart nearly exploded.

“But I’ve got three conditions.”

Three fingers went up.

“One, You gotta tell me everything about this fancy world I ended up in. I’m talkin’ the works. Culture, rules, what the heck I’m supposed to be doing here.”

Yuji nodded rapidly.

“Two, You gotta get me some chili dogs. Preferably two. Maybe three. Actually maybe four. It’s been a long day.”

Yuji blinked. “…Chili… dogs?”

The Uma nodded as if she had just demanded oxygen.

“And three—” She grinned, leaning forward. “Keep up.”

Before Yuji could ask what that meant, the Uma vanished.

Not disappeared, moved.

A blue blur shot down the street, circled back, and stopped in front of Yuji again, all in less than two seconds.

Yuji yelped and nearly fell over.

“Just so you know what you’re gettin’ into.” She winked.

Yuji stared in awe.

That speed. That acceleration. That wasn’t just fast, that was impossible.

But before she could get lost in shocked admiration, she realized—

This was a yes.

“D-DEAL!” Yuji yelled, bowing again anyway. “Thank you so much! I won’t let you down, I swear! Um—w-what’s your name?”

The Uma smirked.

“Heh.”

She put her hands on her hips, proud and confident in a way Yuji couldn’t even imagine being in her lifetime.

“It’s Sonic!”