Summer Pony by Jean Slaughter Doty

Ginny has always dreamed of having her very own pony. So when her parents agree to rent her a pony for the summer, Ginny is thrilled. But when she arrives at the pony farm, the only ponies to choose from are dirty and miserable. All are either too small or too bad-tempered for Ginny. All, that is, except for one. Mokey, a shaggy, half-starved pony, is not at all what Ginny had in mind. Ginny thinks her summer will be ruined! But with a little care, a lot of patience, and the help of some new friends, Mokey surprises everyone. Soon Ginny finds herself wishing the summer would never end.