In Greek mythology, Geryon was the son of Chrysaor and Callirrhoe, and grandson of Medusa. He was a fearsome giant who dwelt on the island Erytheia, in the far west of the Mediterranean, in the company of the mythic Hesperides (Nymphs of the West).
Geryon had three bodies, three heads, and –in later accounts— four wings, all together in one body. He owned a herd of man-eating cattle, whose coats were tinged red by the light of sunset, that the Greek hero Herakles had to capture as one of his twelve works. It was Eurystheus – king of Tiryns— who commanded Herakles to complete these works.
The dreadful cattle were guarded by the two-headed dog Orthus (son of the Echidna) and care for by Eurytion (son of Ares and the Hesperid Erytheia).
Herakles reached Erytheia by sailing across the Ocean in a golden cup-boat borrowed from Helios, the sun-god. There he encountered and slew Eurytion, Orthus, and finally Geryon himself. With this task complete the hero herded the cattle into his boat and led them back to the Greek Peloponnese.
Geryon may originally have been associated with the Constellation Orion, his two-headed dog Orthus with the adjacent canines Canis Major and Minor, and his cattle with Taurus the bull. His father's name Chrysaor, meaning “golden sword”, as often applied to the Orion constellation.
Geryon makes a stellar appearance in Dante’s Inferno. In this text Geryon’s appearance is radically different to that of the Greek myths, for the giant is describe as follows:
“Geryon's face was that of an innocent man,
but his body was half-reptile, half-hairy beast,
with a scorpion's stinger at the end of his tail.”
Then the story tells that poets Virgil and Dante approached the giant and asked him to take them to go and see the sinners in the final round of Purgatory’s Circle VII.
-Chilsholm, A.R., 1929. The Prototype of Dante's Geryon. (Inferno, XVI and XVII). The Modern Language Review, 24(4), pp.451-454.