The Black Stallion's Ghost: Black Stallion, Book 17 by Walter Farley (Paperback)

While on vacation, Alec Ramsay rides the Black down a path into a beautiful but mysterious swamp. There, he discovers a ghostly gray mare and a strange rider. The rider invites Alec and the Black back to his home, but Alec is uneasy. And as that feeling grows, he realizes that his host is not what he seems. Suddenly Alec and the Black are faced with a wild chase through the swamps!