Diomedes kept four savage mares, to which he fed the flesh of unsuspecting strangers. It is said he was as savage as his mares; they were totally uncontrollable and were tethered by chains to a bronze manger.
When Heracles arrived at the palace, the hero took the king prisoner. Then Heracles, knowing the brutality and suffering Diomedes had caused, took the king to his stables and threw him into the bronze manger, whereupon the mares devoured their own master. This caused the mares to be calm and subdued, which made it easy for Heracles to drive them back to king Eurystheus. When Heracles led the mares meekly into the palace Eurystheus, the king dedicated them to Hera, then let them go free to roam the plains of Argos. It is said, that in later years one of the offspring from this breed was Bucephalus the favorite horse of Alexander the Great.
In another version of the same myth, Heracles took with him a group of volunteers, to help with this hazardous task. In the party was a young boy named Abderus, and Heracles was very fond of this youth, and made him his companion. After the hero and his band captured the mares they fled, because they were being pursued by Diomedes and his troops. The Thracian troops soon caught up with the band, so Heracles left Abderus in charge of the mares while he fought off the enemy. Lacking in experience the young Abderus was eaten by the savage beasts. On finding his companion torn to pieces, Heracles, in his anger fed Diomedes to the mares, which then became calm. Heracles gave the youth a worthy burial, then founded the city of Abdera on the site next to his tomb.