it would be awsome if pets cold have like an age we can like upgrade at some point and that at a certain age it changes aperance and size or something. i think it would be epic!
Pets, as they are, automatically upgrade as they level. I think we have enough to do with quests and battles and trying to keep our crews leveled up. I'd rather they didn't add another thing to keep track of.
If you get bored of a pet's looks, try equipping a different one from time to time if you have another, or go farm another one?