The Scorpio Races by Maggie Stiefvater
Puck and her brothers live alone. Their parents are dead, and for money, they do odd jobs--painting teacups, fixing things, anything to get food on the table. When the Scorpio Races come around, Puck realizes that this is her chance to save her family. But is she brave enough to ride one of the wild, carnivorous capaill uisce?
Sean Kendrick is known all over the island of Thisby as the boy whose father died in the Races, who has an incredible connection with all horses, and has won the Scorpio Races five times. When he sees a girl on his beach, he has to know why.