In Greek mythology, the dog guarding Geryon’s cattle was Orthrus.
Orthrus was a monstrous dog usually described as having two heads. He guarded the red cattle on the island of Erytheia, where Hercules traveled during the tenth of the Twelve Labors. Hercules killed Orthrus before taking the herd.
Orthrus is often described as a child of the monsters Echidna and Typhon. In genealogical traditions, he was related to other famous creatures, including Cerberus, the multi-headed hound of Hades. This family connection explains why the two dogs are sometimes confused.
Orthrus was not Cerberus. Cerberus guarded the entrance to the Underworld and belonged to Hercules’ final labor, while Orthrus guarded Geryon’s cattle in the western world. The names and roles are distinct even though both creatures are monstrous canine guardians.