Lara and the moon-colored filly / by Kathleen Duey.

Enhanced descriptions from Syndetics:

The second book in the Hoofbeats series focusing on Lara!

Nine-year-old Lara is the daughter of the the leader of her cattle-raising clan. While she spends her days tending to the cattle, her heart lies with her beloved gray mare. When Lara goes to the highlands to set the cattle out to graze, she finds the mare in the process of a difficult birth. Lara vows to take care of the foal as a dying promise to the gray mare, and with the help of a childless milk-cow, she cares for the spindly-legged filly. But just when she is confident that the foal can survive, a rival clan captures them both, and throws Lara's life into turmoil. When the filly is eventually given to a titled baron in the castle town of Athenry, Lara, determined to stay with the horse no matter what, goes along. Together, she and her beloved horse face seemingly insurmountable challenges, but all along Lara keeps two things in mind. One day, she will manage to flee, and will set off in search of the family that she was taken from. And she will not leave without her silver mare.