Cacus was the son of Hephaestus and Medusa. Inheriting the properties of fire from his father and monstrousness from his mother, Cacus was a fire-breathing monster who was sometimes described as a three-headed giant. He lived on human flesh and decorated his cave with the bones and skulls of his victims.

The most famous myth involving Cacus tells of his encounter with the hero Heracles. For one of his twelve labors, Heracles had to steal cattle from Geryon (another monster) and drive them back to Greece. Heracles successfully stole the cattle and was on his way home when he stopped to rest by a river. As the hero slept, Cacus spotted the magnificent herd and decided he wanted the cattle. He stole four bulls and four heifers. As Hermes had done in another myth (see Chapter 14), Cacus drove the cattle backward to mask their trail.

When Heracles awoke and saw that some of the cattle were missing, he searched for them, but Cacus's trick sent him in the wrong direction. Just when Heracles had almost given up, some of the cattle in his herd mooed, and a cow hidden in Cacus's cave answered their calls.

Realizing that his hiding place had been discovered, Cacus blocked the cave's entrance with a giant boulder. But that didn't stop Heracles; he broke off the mountaintop to expose the cave's interior. Cacus and Heracles fought; Cacus used fire and Heracles used boulders and tree branches. Cacus produced so much smoke that it was difficult for Heracles to see his target. He jumped into the cave where the smoke was thickest. He found Cacus there and strangled the monster to death.