Opossums have been roaming the earth since the time of the dinosaur, thus they are very sophistocated animals (anthropologically, not sociologically) with a prehensile tail and many extra teeth.  These are very misunderstood animals.  Though they look quite fierce, they are very docile animals, playing "possum" when frightened.  They are almost the lowest of the low on the food chain.  They feed on carrion, catfood, and whatever they can get their paws on.  They are nocturnal, probably adapted to have different schedules than most of the other animals who would eat them if they were awake.  Some people think that they are large rats, when really they are sweet animals who want to be ingnored and eat a little bit of your cat's food.  Look out for them if you're driving around at night (if you live in the American South or in New Zealand that is...).
These two lived in my bathroom wall until I told them to get out.