Boris Flathead, the Keeper of the Tomb of the Twelve Flatheads during the tenth century, chained cerberus near the tomb as its guardian. After 948 GUE, when the fierce cerberus had been made friendly by the passing of the Second Dungeon Master, Boris found the three-headed dog to be useless as a guardian. He placed a classified in The New Zork Times attempting to sell him for cheap. Herakles, perhaps heeding this ad, returned cerberus to Hades.

SOURCE(S): Zork: The Cavern of Doom, The New Zork Times 3.1