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It's a horse whose data was deleted from the game server. It's not uncommon for a horse that's on an account that is deleted, either due to the player choosing to delete it or from the company deleting it due to inactivity, to become a "disappeared horse". There's only so much server space available and the company has to pay for it, so they have to purge the data from those deleted accounts periodically to keep the game from getting laggy for the people still actively playing.