Willow King (by Chris Platt) is one of my favorite horse books! A description provided by the library is below.
"Thirteen-year-old Katie, who is herself physically challenged, saves a crippled foal from euthanasia and nurses him back to health and eventual championships."
The library's description doesn't provide much detail, so I'll tell you a little bit more about the book. One of Katie's legs has always been shorter than the other, so she is forced to cope with difficulties at school and frequent visits to the doctor. Katie has a part-time job at a nearby horse ranch, where she discovers that a newborn crippled foal is about to be euthanized. She begs the owners not to euthanize the foal, and her parents reluctantly allow her to "adopt" the foal. Ecstatic Katie names him Willow King, and--I believe that's quite enough for a description! I don't want to give it ALL away, you know. Hope you like it!