In Greek mythology, King Diomedes of Thrace (Ancient Greek: Διομήδης) was the son of Ares and Cyrene, ruler of the warlike Bistones on the northern Aegean coast, and master of man-eating mares. Capturing those mares was the eighth of Heracles' Twelve Labours; Diomedes himself was slain in the struggle. He is distinct from the Argive hero Diomedes, son of Tydeus, who fights at Troy in the Iliad.
Adapted from Wikipedia, "Diomedes of Thrace" · CC BY-SA 4.0
I.Family and identity
Diomedes of Thrace was the son of the war-god Ares and of Cyrene, herself said to be a daughter of Hypseus, king of the Lapiths. He ruled the Bistones, a warlike Thracian people, and kept mares that fed on human flesh, tethered by chains to a bronze manger because ordinary ropes could not hold them.
He must not be confused with Diomedes son of Tydeus, the Achaean champion of the Iliad. Ancient and modern writers mark the Thracian king by geography, parentage, and the mares that define his labour.
II.The Eighth Labour of Heracles
Eurystheus ordered Heracles to bring Diomedes' mares to Mycenae. In Apollodorus' version Heracles sailed with volunteers, overpowered the grooms, and drove the mares toward the sea. When the Bistones came armed to the rescue he left the horses with Abderus — a son of Hermes from Opus in Locris and a companion of Heracles — while he fought. The mares dragged Abderus to death; Heracles slew Diomedes, routed the Bistones, founded the city of Abdera beside Abderus' grave, and delivered the mares to Eurystheus, who released them. They wandered to Mount Olympus and were destroyed by wild beasts.
Other tellings reverse the emphasis: Heracles wrestles Diomedes alone and feeds the king to his own mares, after which the horses lose their taste for human flesh and become manageable. Still other versions have Heracles yoke the mares to a chariot to tame them, or take Diomedes' chariot together with the team. One later tradition claims Eurystheus dedicated the mares to Hera and that a descendant of the line became the horse of Alexander the Great.
III.In art
The labour appears in classical and later art as a combat between Heracles and the Thracian king or as a scene with the savage mares. A well-known Renaissance treatment is Vincenzo de' Rossi's marble group Hercules and Diomedes among the Labours of Hercules at the Palazzo Vecchio in Florence.