As for cats....heh heh....the majority of their quirks are just their instincts coming into action. Then again, there are some cats that get mistreated and are rather aggressive as a result. Some cats remain aggressive and wild-like no matter how much love and attention you give them. This is because cats are not naturally domesticated animals. Humans decided to do so, and now we must deal with the consequences. I've got a consequence on my lap right now: a fat neutered male named Andy who is begging to be petted. And the only reason I say he's male is because of what the surgeons cut off, NOT the way he acts. Believe me, he's all girl!
Aaaaand I can't help but mention declawing.