I think my cat is a monkey. I call him monkeycat, for he likes to sit on my shoulder and climb on trees and make funny noises.
He weighs 12 pounds now and he realized only through failure that he's too large and too heavy to sit on my shoulder, now.
How does your cat think it's a mouse? Does it get caught in mouse traps often? Does it run away from other cats making very annoying high-pitched noises? Does it eat lots of cheese? Was he raised by a mouse?