Wildfire. Zane Grey

Lucy Bostil is the daughter of a big horse lover and she enjoys horse as well, On one of her rides she encounters Lin Sloan during his hunt for a wild mustang he calls Wildfire. When Lin and Wildfire almost die in an accident, Lucy manages to save them, and both the man and the horse grow fond of Lucy. After her father gets involved with some murky characters Lucy gets in trouble and Lin and Wildfire embrace the opportunity to return a favor and save her.