You have to cast the spell because pets that comes with a card either at baby or ancient ect. will be added to your deck. Pets with may cast spells cast their own spell like spritely.
Example: My storm cat got to ancient and gave me the storm cat spell. My starfish got a may cast blade and spritely. My storm cat's card is added to my spell deck for me to cast. My starfish's may cast talents are casted by it.
Cinder, that is true, but what I think the poster was referring to is a graphics bug where the player casts the card instead of the pet...so when you use your storm cat spell, your cat is the one that does the actual casting, not you. Sure, it takes your mana, your pips, and you have to choose to cast the spell, but, looking at it from a purely graphical aspect, the pet is the one that is supposed to cast the item card spell.