Wild Magic — Tamora Pierce

Animal Instinct

Young Daine's knack with horses gets her a job helping the royal horsemistress drive a herd of ponies to Tortall. It soon becomes clear that Daine's talent, despite her struggles to hide it, is downright magical. Horses and other animals not only obey the mysterious girl, they come to her and seem to listen to her words. But Daine will have to learn to trust humans before she can come to terms with her powers, her past, and herself.

Set in Tortall during the reign of King Jonathan III and Queen Thayet, wild Magic is the first book of The Immortals series, which chronicles the time when the world is invaded by immortal creatures and a girl is born with a magical gift that could restore the very balance of nature.