I'd definately be a cat of some sort, a big cat like a leopard or maybe even a house cat. But I'd have to be as pampered as my cat is to be a happy house cat. I haven't decided between the two yet
Proteus anguinus (Otherwise known as Olm). I hide in a cave, get very old (100+ years) and will eventually lead to the discovery of a cure against cancer, though nobody knows how exactly, as of yet. Plus I'm super cute.
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Or a tiger/panther/cheetah because the cat life looks pretty fun.