These are phrased as if talking to players/PCs, but can totally be used for NPCs as well. "Kitten" below could be any animal smaller than a dog, really.
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Every time you learn a spell, a kitten is born from thin air∆. That kitten is both a kitten, a real goddamn kitten that will eat and poop and meow, but it is also your knowledge of and ability to cast that spell. To memorize that spell for the day, you have to spend at least a few minutes with that kitten, petting and playing. In a pinch, two-way scrying will work (IE even if physically apart, if you have some way for the kitten to see you and you to see it, time can be spent in this way). If you can't spend time with that kitten, you can't re-memorize that spell.
If the kitten dies, you don't know whatever spell is associated with it any more.
The kitten has no other abilities beyond that of a kitten. They get 1 HP for every level you have.
All the references to kitten above could be a puppy, mouse, snake, frog, etc - whatever animal is cool with your DM.
Other Magic-Users can try to steal each other's animal-spells. If a Magic-User comes into physical contact with another Magic-User's spell kitten (or whatever), they can ply it with petting, treats, play, etc to try and bond. The "stealing" Mage makes a Charisma roll with their current level as bonus and the level of the kitten's current "owner" as a penalty. (CHA + your level - their level). If this roll is successful, the "stealing" Mage then chooses: the kitten is either bonded to both (both Mages know the spell and can play with the kitten to memorize it) or the kitten is now "owned" and bonded with the "stealing" mage, and the original mage can no longer cast that spell*.
Animal-spells cannot be "stolen" in combat or any other stressful situation. They must be completely comfortable to bond with a new person. They can be stolen in the mundane, physical sense, of course.
This would not work in every game, and would require a bit of management and commitment. Fun though.
Perhaps Rogue-types might be able to "read" spell-kittens like a one-use scroll?
A Magic-User can try to determine what spell an animal bonded to someone else stores by concentrating on observing it for one round. During that round, the Magic-User makes an Intelligence check, modified by the difference in level between them and the animal's "owner". If successful, they know what spell that animal "stores".
Make sense?
∆ or perhaps you only learn spells by stealing other Magic-User's animals, and new ones are only made via magical research with all the time and cost that entails.
*EDIT: I'd probably give the kitten's original "owner" some kinda roll to fight this, maybe.
EDIT EDIT: Spike posted some great stuff in the comments below, check it out!
