Ok so I'm sorry if this sounds stupid, but I'm a little confused. I was doing a trade with another player, but I got off. They sent me the creatu, but didn't get their payment and asked for the creatu back. If their account got banned, I still get to keep the creatu, don't I?

It depends on the situation. You'll need to open a Support Ticket and let us know who the user was, the pet in question, etc. Only in a ticket though, don't post that info here please. =)