The Pomona was allegedly an early-19th-century sailing ship.
According to a version of his life story once told by the Ghost Host to a Treasure-Hunter, he, as William Gracey, made his fortune under the pirate alias of Captain Blood sailing the Pomona. He'd previously worked on the ship as its first mate when it was the property of Captain Randall Pace, a mostly-honest tradesman who later became the Hatbox Ghost.
Behind the scenes
The Pomona first appeared in the first chapter of the Mystery of the Manse miniseries, simply called Part One. It is implied to be the subject of the Flying Dutchman painting in The Haunted Mansion, which was present as early as 1969.