A Horse for the Summer by Michelle Bates 2003 Usborne Books Sandy Lane Stables.

Tom’s cousin has agreed to lend him Chancey, her stunning chestnut gelding, for the holiday. But Tim’s elation turns to dismay when he sees the state Chancey is in. It will take a lot of hard work to get the horse ready for the summer shows, but Tom is determined… Until Nick decides Chancy is too dangerous to ride. What is Tom going to do?