The fire-breathing giant killed by Heracles after stealing cattle was Cacus.
Cacus belonged chiefly to Roman mythology and lived in a cave on the Aventine Hill, in the landscape that later became Rome. While Heracles was driving cattle taken from the giant Geryon, Cacus stole eight animals—four bulls and four cows—and dragged them backward into his cave to conceal their tracks.
The trick failed when the remaining cattle called out and the stolen animals answered from inside the cave. Heracles attacked, but Cacus blocked the entrance with a huge boulder. In Virgil’s Aeneid, Cacus fought by breathing fire and smoke before Heracles reached him and killed him by strangulation.
Cacus is often confused with Geryon. Geryon was the multi-bodied cattle owner whom Heracles defeated during his labor; Cacus was the local thief who stole part of the herd afterward. Roman tradition linked Heracles’ victory over Cacus with the Ara Maxima and the cattle market called the Forum Boarium.