I think you're either talking about deviants or custom skins. For custom skins, you'll need a Rainbow Jar (premium item, bought w/ gold coins, which are bought w/ actual money) as well as paying a fee for using a custom skin (pretty sure some are either free or really cheap, though). Deviants can be bred or bought, but wild pets can't be deviant (for how to breed and chance of success, visit FAQ). DO NOT USE MUTAGEN. You can't get deviance w/ mutagen but CAN lose it this way. If you buy one, they're usually pretty expensive. Check FAQ, it's really helpful.