The Black Stallion's Ghost (Black Stallion) (original 1964; edition 2002)

ALEC RAMSAY AND THE BLACK - LOST IN THE EVERGLADES AT NIGHT!

Not since those first days together on the desert island had Alec and the Black been so free. Then one morning, during their vacation, Alec led the Black down a path into a beautiful but mysterious swamp. And what started as an innocent ride turned into a living nightmare. Alec could no longer tell what was real and what wasn't!

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After the grueling races at Miami's Hialeah Park, Alec Ramsay and the Black were taking a well-earned vacation on a Florida ranch. When Alec rode the Black into the Everglades one lazy morning he scarcely dreamed that this would be one of the most terrifying days - and nights of his life.
In a clearing he met a tall, mysterious black, man riding a gray mare. 
Alec could see from the way the man put his mare through the steps of the huate ecole - the high school of riding - that he was a professional horseman. So when the man invited Alec to have lunch at his home, Alec couldn't understand his feelings of uneasiness. But as the day turned into night and he learned more about his host, Alec's fears grew ....