Have you ever noticed that some fish like the Ruffy transforms the pet into a Griffin has a magic skill level at 100 but it cannot cast spells? Well just try feeding the pet a spell like you would a potion, by dropping it into the pets toolkit up at the top corner of the screen. Note that shift clicking will feed a pet a potion but not a spell. You can teach a magic pet a spell this way. Griffins, Brainbeasts, Abominations, and the Dire Unicorn all come with a magic skill of 50+. Yet only the Brainbeast and Abomination can natural cast spells. A few Notes on teaching pets spells.
1. You can only drop a spell into their feeding box if your hero has magic enough to write into his/her book.
2. They cast spells at random and only with monsters around, so your going to have to wait a little while to see if your pet can cast the spell you just feed it.
3. It can learn any spell feed to it regardless of its magic skill level but it can only cast those with magic requirements low enough to do so.
I just found this out today after 15 or so years playing the game on and off. Hope this helps and have fun teaching your buddy to summon bats, or heal itself. or throw fireballs into the fray.