This is a book by Angela Dorsey from her collection called Horse Angel. It is one of my favorite horse books and it is very exciting read.
This is from the back of the book:
While Chance and her best friend Hanna are out horseback riding, with Chance's mare's foal in tow, disaster strikes in the form of a deadly tornado! Everything goes wrong - the girls are separated, their school rivals are out to get them, the foal disappears and then... it gets worse!
But then a strange girl appears, with pale skin and hair of gold, who seems to possess magical powers to help the horses and their riders, Will even she be able to wrestle with the forces of nature?
A gripping tale!