Home at Last (Marguerite Henry's Misty Inn, Bk. 8)

Even though Ben Dunlap is allowed to take care of his older sister Willa's pony, everyone knows Starbuck is really hers - the two are inseparable. But then Ben and Willa discover a wild, renegade pony, and Ben wants to keep him. He even names the pony Winesap. Willa worries Ben will grow too attached, and she's right. Just like that, Ben and Winesap become the best of friends, and Ben won't give him up.