I think that pet spell cards that come with pets should be able to be cast as mastery spells (using power pips). Especially the high level spells.
I have a Crop Watcher which I got when hatched with my Judgement pet and Forest Lord pet. The Crop Watcher pet has talents useable by my Balance character, but the spell it comes with is an Ominous Scarecrow spell card which is an 8 pip spell. Unless I buy the Death Mastery Amulet (almost 10,000 crowns) which is a waste for casting one spell, my character will never be able to cast this spell.
Why not offer something at the Pet store (or crown shop) call it say a Pet Collar or Pet Leash ?
This would be connected to a specific pet and allow the character using the Pet to cast its spell using Power Pips. You could do a simple graphic on the Pet information page saying that the pet has the collar or whatever it ends up being.