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Before our story begins, here are some facts that you should know about pigs.
There are different breeds of domestic pig, just as there are different breeds of dog and cat. Some are bred for the show pig industry, others for companionship, and still others for market -- which is to say, for their meat.
When you win a pig from a farmer at a carnival and he hands you a knife and fork with which to eat it, that means that your pig is a market pig.
Market piglets are weaned at about twenty-one days of age, whereupon they can be taken away from their mother and offered as carnival prizes. Just-weaned piglets typically weigh from thirteen to fifteen pounds.
One of the reasons that pigs are so highly prized as a food source is that they grow quite quickly. Females (at least those bred for market) are ready to reproduce within 170-220 days of age. It takes about six months for a market-bred piglet to reach its full size. A pig that's ready for market weighs around two hundred and eighty pounds.
It is also a known fact that an animal that is loved very much by a young girl can grow to larger-than-normal size, as seen in the documentary Clifford the Big Red Dog.
Pigs are highly intelligent -- smarter than nearly any other common domesticated animal, including dogs. They have keen memories, and those who have kept them as pets report that they require firm obedience training, as anything one's owner permits even once will stick in its mind as something I'm allowed to do. They can become aggressive if (for example) one's owner finds it amusing to let it attack nearby teens who happen to have candied apples, and allows it to do so without consequence. They can hold grudges against specific individuals, should they have a negative encounter with one. They can also exhibit aggressive behavior in response to stress if sudden changes occur within their household.
Another product of their intelligence is that they have the capacity to get bored when left alone for extended periods of time, which often leads to destructive behavior. Such behavior can include transforming a freshly-mowed lawn into a mess of overturned soil, or eating a large amount of unattended corn.
Pigs are also just generally kind of jerks. Their nature drives them to continually challenge their herd mates in order to ensure that the strongest herd member is leading them and keeping them safe. If they happen to share a household with a dog, this overwhelmingly results in especially disastrous consequences (usually for the pig).
While pigs are not as unhygienic (compared to other animals) as fiction often portrays them, they are notoriously messy eaters.
Pigs are prone to certain health problems if not raised under specific conditions. Obesity, for instance, is a major concern if one is overfed or given too many carbs or high sugars. This can lead to further problems, such as arthritis. Veterinary care for pigs is incredibly expensive.
With all of this in mind, it's genuinely amazing that Mabel and Dipper's parents put up with Waddles for as long as they did.