Originally a pre-Roman god of fire, who gradually became a fire-breathing demon. Cacus lived in a cave in the Aventine Hill from where he terrorized the countryside. When Heracles returned with the cattle of Geryon, he passed Cacus' cave and lay down to sleep in the vicinity. At night Cacus dragged some of the cattle to his cave backward by their tails, so that their tracks would point in the opposite direction. However, the lowing of the animals betrayed their presence in the cave to Heracles and he retrieved them and slew Cacus. Other sources claim that Cacus' sister told Heracles the location of his cave. On the place were Heracles slew Cacus he erected an altar, where later the Forum Boarium, the cattle market, was held.