Much as I love cats, when it comes to companions from an anime I’ll have to go for Yakkuru from (you guessed it!) Mononoke Hime.
Yakkuru is Ashitaka’s red elk friend. I hesitate to call him anything else because to my mind that’s always what he’s been. He’s loyal and always looks out for Ashitaka, even to the point of putting his own safety at risk.