The Haunted Horseman was the disguise of the Duke of Strathmore.
The Haunted Horseman was a phantom knight who wore pale white armor and carried a lance. The duke used luminous paint to make himself and his horse look ghostly.
He rode through the corridors of Hagglethorn Hall, scaring and chasing people.
Powers and abilities
He used secret passages in the castle to make it appear that he could go through the walls.
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The gang first saw the Haunted Horseman when asking for directions to Hagglethorn Hall at Cyrus Wheedly's farm. The farmer warned them about the ghost, who rode past the farm at that moment, knocking the pumpkin head off of the scarecrow with his lance. Despite the warning, they still went to Hagglethorn Hall and offered to help solve the mystery of the Haunted Horseman, who was scaring tourists away from the castle.
While searching, Shaggy Rogers, Scooby-Doo, and Fred Jones heard hoofbeats, which were being made by none other than the Haunted Horseman. He chased them into a room where they closed the door, locked it, and barricaded it, leaving the only way in being to go through the wall. However, the Haunted Horseman did exactly that and got into the room. They removed the barricade, ran out of the room, and closed the door. There they found the butler, Creech, who wondered what was going on. They told him it was the Haunted Horseman, but when Creech looked into the room, the ghost was gone. Then Creech disappeared, and the Haunted Horseman came back. They fled into a different room and closed the doors, holding them shut. Scooby saw the Haunted Horseman come out of the fireplace, prompting him to leap up and grab onto a chandelier. Shaggy and Fred ran out of the room, while Scooby slipped off of the chandelier and landed on the ghost's horse. He fell off, and Shaggy and Fred came back and picked him up. They got away by hiding in some pots.
They found the Haunted Horseman again, who chased them into a dead end. However, they were saved by Creech, who pulled them through a secret panel in the wall. They discovered the secret to the ghost's apparent ability to pass through walls; he used secret passages.
Later at Cyrus Wheedly's farm, the Haunted Horseman was caught when his lance got caught on a rope, causing hay bales to fall on him and knock him off his horse. The ghost was unmasked, revealing the Duke of Strathmore, who wanted to take over Hagglethorn Hall, dismantle it, and sell the bricks for souvenirs. He also had help from Cyrus Wheedly, who dressed up as the Moat Monster in exchange for half the money from the sale of the castle. After the two were caught, instead of calling the authorities (the duke pointed out that doing this would bring great shame to the family name), the two had to use their disguises to attract visitors and help raise money for Hagglethorn Hall. The duke had to dress up as the Haunted Horseman and ride through the halls three times a day, giving him saddle-sores.