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.