[ZenGum]

It's hard to chose a favourite.
So here are a few thoughts with reasons.

Dogs and cats, of course, because they can from social bonds with humans, fairly intelligent and friendly.

Chameleons, because some of them fight by flashing bright colour changes at each other. Some squid do that too.

Koalas and wombats - endearingly cute, but otherwise not much.

Eagles and owls - I wish could transform into one at will (and back, though).