A Giant King in Gades once did keep,
(Unlike their gentle race) carnivorous sheep,
The triple monster slain, amidst his rocks
He left to birds a prey, and seiz'd his flocks,
Which by Eurystheus' order brought away
He unto Argos safely did convey.
In Greek mythology, Geryon was a fearsome titan who dwelt on the island Erytheia of the mythic Hesperides in the far west of the Mediterranean. Heracles was required to travel to Erytheia, in order to obtain the Cattle of Geryon as his tenth labour.
When Heracles reached Erytheia, no sooner had he landed than he was confronted by the two-headed dog, Orthrus. With one huge blow from his olive-wood club, Heracles killed the watchdog. Eurytion the herdsman came to assist Orthrus, but Heracles dealt with him the same way.
On hearing the commotion, Geryon sprang into action, carrying three shields, three spears, and wearing three helmets. He pursued Heracles at the River Anthemus but fell a victim to an arrow that had been dipped in the venomous blood of the Lernaean Hydra.
Heracles then had to herd the cattle back to Eurystheus. To annoy Heracles, Hera sent a gadfly to bite the cattle, irritate them and scatter them. The hero was within a year able to retrieve them. Hera then sent a flood which raised the level of a river so much, Heracles could not cross with the cattle. He piled stones into the river to make the water shallower. When he finally reached the court of Eurystheus, the cattle were sacrificed to Hera.