Adding that to existing pets might get OP, and could be a lot of programming, but introducing new pets with that feature shouldn't be too hard. There could be a normal pet, but instead of coming with a card, it comes with a triangle socket. You could install any triangle jewel in that for an extra card or stat, but once you do it the pet becomes no trade (just like any item with a jewel). That would prevent OP jewels from being moved to low level wizards. If you hatch the pet, the baby has the socket but no jewel, so each can be different.