More an RPG question, but: Do you think a halfling (36" high; 35 lbs) could ride a spotted hyena (28-36" high at shoulder; 150 lbs, per Wikipedia)? I'm assuming she could train it via magic, but I'm not sure its size and physical structure are suitable...
(I’m assuming a lot of D&D 3.5 edition rules because those are the one’s I’m most familiar with)
A halfling could certainly get a saddle made to sit on a hyena, but a standard hyena is probably approaching its carrying capacity and the limit of its strength to carry a halfling around for any length of time, and that’s not accounting for gear and armor.
You could potentially modify the hyena, adding a couple of hit dice to increase it’s strength score (and possibly height) so that it can carry more for longer. It’s only a small tweak that would be needed in the rules. Domesticated hyenas bred for increased carrying capacity is an interesting possibility.
Dungeons & Dragons often pairs halflings with riding dog’s, so I don’t see why you couldn’t modify those rules a little to suit your idea. It is fantasy after all.