Some are from cash shop items; all can be bought from user shops/trades/auctions. Newest one was in the Token Shop briefly.

Check SubetaHQ for a list; check in Layers for Background and Foreground. Some of the animals are from different layers, like Held, etc.

and the backgrounds are "backdrops" xD

Here is a thread with links to wearable animals, etc., that should help you a lot.

They are just items ^_^ Like regular clothes, but you don't wear them.. you do wear them but they show up as a backdrop like a bracelet as a bracelet