I've bought a few pet's, but I've found they don't give much, only a few give cards. I've come up with an idea. They could be used as a form of transport (Ride on a unicorn, ride on a firecat, fly with a dragon, etc.)
Do you know how much work that contains? It could take long because they would have to make sure a unicorn could move in every spot. This is not a mob game not a regular virtual world. You do ride on this but they are not your pets later on in the game.
Maybe they could sell something at the Pet Shop that you can add to your pet. I bought my dragon (Jasper) early and nothing comes with him at all. I would love it if he added percent pip increase or a card or adds tough or strength or accuracy. I would pay for that because I do not want to part with him.