The cattle of Geryon was the fourth labor of Heracles
As a fourth labor, Eurystheus ordered Heracles to go to the island Erytheia and bring him the cattle of the monstrous Geryon.
Heracles went through a lot of trouble to reach the island that lay in the very west of the Mediterranean, near the Ocean (in today's Spain)...he was threatened by savage animals and overheated by the sun until, out of frustration, he aimed his bow against the god of the Sun, Helios. The Sun god became so impressed by the hero, that he borrowed him his golden cup in order to sail safely across the sea.
When he reacehd the island, it proved to be hard to approach the cattle because they were guarded by the herdsman Eurytion, a son of Ares, and Orthos, a dog with two heads and a snake tail.
But Heracles managed to defeat the dog with his club and kill the herdsman as well. While trying to drive the cattle away, he confronted their master Geryon who engaged him in a battle. With an arrow, he killed Geryon as well.
So Heracles put the cattle in the golden cup , crossed the sea and then returned the cup to the Sun-god. In his remaining journey, Heracles went through even more turbulences, which he skillfully mastered , when finally he reached Mycenae and handed the cattle over to Eurystheas. The king offered the cattle in sacrifice to Hera, the Queen of the gods.