In the Twelve Labors of Hercules, Geryon’s two-headed watchdog was named Orthrus.
Orthrus guarded Geryon’s red cattle on the western island of Erytheia. He was commonly described as a two-headed dog and as the offspring of the monstrous pair Typhon and Echidna. His brother was Cerberus, the better-known watchdog of the underworld.
During the labor, Hercules killed Orthrus before taking the cattle. The herdsman Eurytion also attempted to protect the animals, and Geryon himself later confronted Hercules.
Orthrus is frequently confused with Cerberus because both are supernatural guard dogs connected with underworld monsters. Cerberus guarded Hades and is usually represented with three heads, whereas Orthrus belonged to Geryon’s cattle herd and is usually represented with two.