Possibly a fire god or perhaps simply a deity associated with a place, Cacus was one of the heroes local to Rome and his myth is closely linked with that of Hercules. He is said to have been the son of Vulcan, and lived in a cavern on the Aventine Hill. When Hercules returned from his expedition in the western Mediterranean, he brought with him the oxen which he had stolen from Geryon; he left them to graze freely on the place which later became the Forum Boarium while he rested by the banks of the Tiber, and Cacus, though he could not steal the whole herd, much as he wanted to, removed a number of the beasts (said to be four cows and four oxen) and concealed them in his cave. In order to leave no clues he pulled the beasts by their tails, making them walk backwards, so that their tracks seemed to be coming away from the cave and not going towards it. When Hercules awoke and counted his herd he realized that a thief had been at work; he went in search of his possessions and would have been taken in by Cacus's trick if, as some accounts say, the beasts had not lowed when they heard the rest of the herd and thus given away where they were, or if as others maintain, he had not been told what had happened by Caca, Cacus's sister.

Whatever the true version, Hercules and Cacus began to fight. Cacus had three heads and blew flames from his three mouths, but Hercules soon got the better of him with his club. In another version of the story Cacus shut himself up in his cave, piling up rocks in front of the entrance to resist the assaults of Hercules, but the latter climbed the hill and tore away the rocks which formed the roof of the cave; he succeeded in reaching his enemy and killed him. Then he offered a sacrifice to Jupiter Inventor, in gratitude for his success. King Evander, then reigning in Pallanteia (the site which later became Rome but was then merely a village on the Palatine hill, inhabited by shepherds) thanked Hercules for having disposed of such a robber as Cacus and promised Hercules that heaven would reward him by conferring divine honours on him.

An obscure version of the legend about Hercules substitutes a robber called Garanus or Recaranus for Cacus. According to one early Roman historian, Cacus was a comrade of King Marsyas, who had come from Phrygia to invade Italy. Marsyas sent him as an ambassador with the Etruscan king Tarchon, but he was taken prisoner by the latter. Cacus managed to free himself and returned to Marsyas. Marsyas and Cacus then seized Campania, the country named Volturna and attacked the region of Rome, where an Arcadian colony had established itself. At this point, however, Hercules allied himself with Tarchon and crushed the invading force.

Finally, Diodorus writes of one Cacius (Καχιος) who was a man of exceptional strength. He lived on the Palatine and had entertained Hercules with hospitality. This Cacius gave his name to a rise on the Palatine known as the Scalae Caci (Cacius's steps) near his house (atrium Caci).

{E2 DICTIONARY OF CLASSICAL MYTHOLOGY}

Table of Sources
- Virgil, Aen. 8, 190ff. with Serv. ad loc.
- Livy 1, 7, 3ff.
- Dion. Hal. 1, 39ff.
- Ovid, Fast. 1, 543ff.; 5, 673ff.; 6, 79ff.
- Prop. 4, 9, 1ff.
- Tzetzes, Hist. 5, 21
- Serv. on Aen. 8, 203, citing Valerius Flaccus
- Solinus 1, 8
- Diod. Sic. 4, 21