Flicka Series by Mary O'Hara

It seems Ken can't do anything right. He loses saddle blankets and breaks reins...but then comes the worst news yet: a report card so bad that he has to repeat a grade. How can you tame the dreamy mind of a boy who stares out of the window instead of taking an exam? Enter Flicka, the chestnut filly with a wild spirit. Over the course of one magical summer, both will learn the meaning of responsibility, courage, and, ultimately, friendship.

A follow-up to the childhood classic My Friend Flicka. "That rare achievement, a sequel to a great and richly deserved success that in no way falls short of its distinguished predecessor . . . a fine and singing story."--New York Times

In the third book of the "Flicka" trilogy, Ken discovers to his horror that Thunderhead is roaming wild, causing trouble wherever he goes. He also has trouble at home, and thinks that his dreams can never come true; but then his feelings for Carey begin to change.