Desert Song

Sophie has lost count of all the foster homes she's lived in. Finally, at Joel and Kalene's ranch, she feels at home. When she unwittingly puts their most valuable mare, Aria, in danger, she is terrified she'll be sent away and conceals her mistake - and Aria - letting her foster parents believe the mare was stolen.
Then Sophie is hit by a car. She knows she must go to Aria as soon as she can, though she's not sure how she's going to carry hay with a broken arm.
But feeling hungry is not Aria's biggest problem, for a terrible evil has found her and knows she is alone.