Cacus was one of the local deities or heroes of Rome. His legend is closely connected to that of Hercules. He was believed to be the son of Vulcan, had three heads, and could breathe out fire; he lived on the Aventine Hill in a cave. When Hercules was returning from his expedition in the region of the present Spain, he brought with him the oxen, which he had stolen from Geryon. He left them graze freely, while he himself was having a rest. Cacus stole some of the oxen and hid them in his cave. When Hercules found that out he began to fight Cacus and won the battle.
See: Nicolas Poussin Landscape with Hercules and Cacus.