Summer of Wild Hearts — Angela Dorsey

At last, Evy's over-protective, hermit mother is letting her go somewhere - to the rodeo in town! She plans on having tons of fun doing something normal for a change. But as usual, adventures find her, this time in the form of an abused, drugged, and terrified mare who hates humans so much she refuses to trust even Evy. As Evy tries to help her, Twilight, her intrepid filly, is miles away and in danger. How can Evy save them both?