The Eighty-Dollar Champion by Elizabeth Letts
I'm a sucker for books about dogs or horses. This time it was a horse. Snowman was a reject at a horse auction and headed to dog food and glue when Harry De Leyer bought him for $80 as a lesson horse for a girls' school. Discovering the horse's ability to jump fences, De Leyer began the training to make Snowman a champion jumper. In 1958 and 1959 he took away national honors outperforming the usual thoroughbred competition.
Letts writes a great story about this unlikely rags to riches story about a horse.