The Labours of Heracles are amongst the most famous stories from Greek mythology, and the 12 tasks assigned by King Eurystheus are legendary. Some of the 12 labours are more famous than others, and indeed the capturing of the Mares of Diomedes is one that many readers forget.

The Mares of Diomedes belonged to King Diomedes; King Diomedes was the ruler of the Thracian Bistonian tribe, a people considered to be a barbaric people. Diomedes himself was an offspring of the god Ares and nymph Cyrene.

This Diomedes should not be confused with the Achaean hero of the same name, who fought at Troy, and was one of those hidden inside the Wooden Horse.

The Mares of Diomedes were special, but not in the same way as the Ceryneian Hind. King Eurystheus did not want them to possess them, as he had done with the Hind, but allocated the task to Heracles in the belief that the hero would be killed in the effort.

What made the Mares of Diomedes special was the fact that the horses were maneaters.

The horses of Diomedes were fed human flesh by their owner, the flesh normally coming from the bodies of the unsuspecting wanderers who traversed the Bistonian territory. There were generally considered to be four mares of Diomedes, and the Roman writer Hydinus, in Fabulae, named them as Deinos, Lampon, Podagros and Xanthos. 

An additional danger that would face Heracles, was the fact that Diomedes was the son of a god who was quick to anger, and so any harm that befell Diomedes could be revisited on Heracles.

Not daunted by the dangers ahead, Heracles had of course already faced the Nemean Lion and Lernaean Hydra, the hero set sail with a team of companions for Thrace.

Heracles’ arrival in Thrace was unexpected and the handful of guards that looked after the four mares were no match for the demi-god. The guards were easily driven off, and Heracles quickly tethered the four mares, despite their immense strength. Heracles, with mares in tow, then set off for his ship.

The return journey though was relatively slow, and by then an alarm had been given. Heracles was soon overtaken by Diomedes and a large Bistonian fighting force.

Again, Heracles was not daunted by being outnumbered in a fight, and with the mares having been relatively placid so far, gave their leads over to one of his young friends, Abderos.

Free from restrictions Heracles now started to fight the Bistonians, killing many, until he came face to face with Diomedes. Diomedes though was not a match for Heracles, and the king, like his soldiers, was quickly killed. Any remaining soldiers fled from the field of battle after the death of their leader.

The battle had gone well for Heracles, but when he returned to the mares, he found that they had reverted to their aggressive selves, and Abderos had been killed and eaten by the four horses. Mourning his young friend, Heracles would establish the Thracian town of Abdera in his memory; the town of Abdera would develop into one of the wealthiest in the whole region.

Heracles though was once again faced with the problem of taming the mares of Diomedes, and in an act of poetic justice, this the hero did by feeding the flesh of Diomedes to his horses. Once the four mares had fed upon the flash the unnatural desire for human flesh departed from the creatures.

It was then an easy task for Heracles to return to the court of King Eurystheus with the mares of Diomedes. King Eurystheus though was still fearful that the horses might revert to their man-eating ways, and so rather than keeping them, he bade Heracles to set them free.

The horses roamed free until they came to graze upon the foothills of Mount Olympus, and although some of the horses used by the heroes at Troy were descended from the four mares, the original four horses were killed by the other wild beasts to be found on Mount Olympus.