Diomedes was the king of Thrace and son of the god of war Ares.
Ares had given him as a present, four man-eating mares.
It was said that every stranger that visited Diomedes palace, was thrown as food for the horses.
Those four man-eating mares where to be Hercules seventh labor.
He was to face first Diomedes, then capture those mares and bring them back to Eurystheus as he had asked him to do. Along with Hercules to help him to his labor, was his dear friend Abderus.

After a long journey they arrived to the palace of king Admidus of Thesale. Admidus was cursed by a god to die young. He could be saved if only his wife, or one of his parents took his place on the death bed. At the time of Hercules arrival, Admidus was about to die. Alkisti, his beautiful wife had took his place to save the king.

Hercules was so touched by the sacrifice of Alkisti, that he entered the room at the moment that Death was about to take her soul. Hercules fought Death and defeated him to save the beautiful queen.

So Hercules continues his journey to reach the palace of Diomedes. At the palace his was welcome by the king of Thrace. Diomedes had no idea who Hercules was and his plan was to throw him and his companion to the mares as food. Hercules guessed Diomedes intentions and when they were attacked by the palace guards they defeated them.

Diomedes attacked Hercules, and as he was Ares son and very strong they fought for a long time. At the end the two men fighting they reached the mare's stables. Hercules then thrown Diomedes towards his mares. The man-eating horses killed and eat Diomedes at once.

Hercules manage to capture the wild mares and put them into chains.
His joy was so big for this accomplishment that he turned over the wild horses to his friend Abdirus to hold and lead. Abdirus had not had the strength of the mighty
hero and could not hold the man-eating mares.
He was soon killed by them, and Hercules had to capture them for a second
time before they were lead back to Tiryns and be given to Eurystheus.

But Abdirus was dead... Hercules founded the city of Abdera beside his dear friend's grave.

Eurystheus after receiving the wild horses he let them free in mount Olympus. Some of them where eaten by wild beasts and others calm down. There breed reached the days of Alexander the great. It's been said that the famous horse Voukefalas of Alexander the great was from the same breed of Diomedes mares.