Mister Boots — Carol Emshwiller

Bobby Lassiter has some important secrets--but it's not as if anyone's paying attention. It's the middle of the Depression, and while Bobby's mother and older sister knit all day to make money, Bobby explores the California desert around their home. That's how Bobby finds Boots. He's under their one half-dead tree, halfdead himself. Right away he's a secret, too--a secret to be fed and clothed and taken care of, and even more of a secret because of what he can do. Sometimes Boots is a man. Sometimes he's (really, truly) a horse. He and Bobby both know something about magic--and those who read this book will, too.