Avalon: Web of Magic Book 7, Song of the Unicorns by Rachel Roberts

Emily's dad wants her to visit and meet his new wife at a horse ranch in New Mexico. Emily convinces Adriane and Kara to accompany her. But along the magic web, a dangerous magic tracker is hunting down unicorns. In the deserts of New Mexico, the girls discover an entire herd of baby unicorns, which they must hide from the tracker before it is too late.