More on same... This is dredged out of the cesspit of my brain, but should be mostly accurate.

First, pigs are very similar to humans biologically. Tissue similarity is strong enough that replacement heart valves are often "harvested" from pigs (this doesn't do much for the pig, note). The flipside is that the same similarity benefits organisms, particularly parasites and disease organisms. Hence: biblical (and other cultures' ritual prohibitions) on eating pork, swine 'flu, and various intestinal worms. Maintaining household animals of slightly greater special differentiation reduces the risk of disease transmission. Fleas are the most typical problem, generally controlable.

Pigs are big. Even the potbelly pigs which were popular a few years back (I actually saw one, kept by a couple in their RV trailer, at the Stanford University Hospital parking lot, where the woman was staying following her heart-lung transplant as she battled cystic fibrosis, and where I was working for the man who's now FDA administrator...another of those stories you just can't make up). Cats range from 5-25 lb, with 10-15 being the usual range. Dogs cover more ground, but still tend to be smaller than your typical pig. I've never seen a yippy pig (though I suppose they probably exist...). Hmm, maybe not:

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The pigs kept as pets in Australia are dubbed Australian Companion Pigs or Miniature Pigs. However, the term miniature is misleading given they still grow to weighty proportions in pet terms, that is, up to 100kg (sometimes more) and need a fair amount of room to move. Their weight however is considerably less than the average pig bred for the kitchen table which can range between 350 and 500kg.

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[link|http://www.abc.net.au/creaturefeatures/facts/pigs.htm|Creature Features]

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Pigs are aggressive. In addition to the Hawaii feral pig stories below, there are periodic tales of attacks in the SF Bay Area, notably Marin, which has a fairly substantial population. Worse during whatever pig rut is called. Pig farming is a dirty business (been to pig farms in Iowa), in part because pen populations are kept large enough that pigs can't keep track of who's who -- in order to reduce the likelihood of fights. Pigs are also omnivores, and will pretty much go after anything when hungry. They're not quite the hunters cats and dogs are, but they are opportunists.

Pack behavior of dogs usually manifests itself as treating the human as the "top dog". Cat behavior is different, I believe Ben's posted here about domesticated cats essentially regressing -- an adult domesticated cat is more a kitten (making the owner the mom/dad cat) than a pack animal. My own experience seems to bear this out. I don't know pig sociology in particular, but I suspect they're somewhat herd/packish with "top pig" structures. Not sure how this translates to domestication. Interesting source on much of this (characteristics of domesticated animals, including temperment) is in [link|http://www.wwnorton.com/catalog/spring99/gunsgerms.htm|Guns, Germs, and Steel].