Wild Magic by Tamora Pierce
This was probably my second re-reading of this series and I loved it just as much as I had the first time.
Daine is a young girl, newly orphaned who applies for work at a horse fair. The horse trainer, Onua doesn't want to take on such a young girl but gives her a chance and sets her in among the horses she has already purchased. It's not a problem for Daine that those horses are wild, untamed and unruly because Daine has wild magic and can speak to the animals.
Onua takes her on and they set off for the capital of Tortall because Onua is the horse-mistress of the Queen's Riders. Daine makes herself invaluable on the way by keeping the horses in line and fighting off immortals.
She meets Numair who plays a huge role as her teacher in the rest of the series. Numair is 27 at the start of the series and Daine is 13 so there is a huge age gap there.
Daine's magic grows leaps and bounds under Numairs tutelage and Daine's wild magic becomes instrumental in saving Tortall from Carthaki invaders.
Daine is probably my favorite character in the Tamora Pierce world of Tortall. She's got this natural thing with animals that I would love to have... She's not overly stubborn (like Alanna) or boring (like Kel - she was my least favorite heroine) or mischievous (like Aly, who is my second favorite (^_^) ) or even determined (like Pierce's newest heroine Bekka). Daine is just nice and has a lot of common sense and is brave.
Next up is Wolf-Speaker.