In Greek mythology, Cacus was a fire-breathing giant.
On the Aventine Hill in Italy, Cacus stole some of the cattle stolen from Geryon as the thief, Heracles, slept. He made the cattle walk backwards so they left no trail. Heracles drove his remaining cattle past a cave, where Cacus was hiding the stolen ones, and they began calling out to each other. Heracles killed Cacus. Alternatively, Caca, Cacus' sister, told Heracles where he was.
In ancient Roman mythology, Cacus ("bad") was a fire god. He was later demoted to the giant described above. According to the Romans, after Hercules (the Roman Heracles) killed Cacus, he founded an altar where later the Forum Boarium, the cattle market, was held.
