Mares of Diomedes
The Mares of Diomedes were four magnificent, wild, uncontrollable, man-eating horses. They belonged to the giant Diomedes, King of Thrace, a son of Ares and Cyrene who lived on the shores of the Black Sea. Bucephalus, Alexander the Great's horse, was said to be descended from these mares.
Heracles was not aware that the horses were kept tethered to a bronze manger because they were wild, man-eating and uncontrollable, and Heracles left Abderus in charge of the horses while he fought Diomedes, but Abderus was eaten. In revenge, Heracles fed Diomedes to his own horses, then founded Abdera next to the boy's tomb.
In another version, Heracles stayed awake so that he didn't have his throat cut by Diomedes in the night, and cut the chains binding the horses. Having scared the horses onto the high ground of a peninsula, Heracles quickly dug a trench through the peninsula, filling it with water and thus rendering it an island. When Diomedes arrived, Heracles killed him with an axe (the one used to dig the trench), and fed the body to the horses.
Eating made the horses calmer and Heracles took the opportunity to bind their mouths shut, and easily took them back to King Eurystheus, who dedicated the horses to Hera. In some versions, they were allowed to roam freely around Argos, having become permanently calm, but in others, Eurystheus ordered the horses taken to Olympus to be sacrificed to Zeus, but Zeus refused them, and instead sent wolves, lions, and bears to kill them.
According to the Trojan War epics, King Diomedes, the hero of Argo, who fought against the god Ares during the war, stole horses from the stables of King Rhesus of Thrace. Since Herakles was the hero of Argo, and since Diomedes (of Argo) was perceived as already owning the horses, Diomedes had to take the place of Rhesus in the story, becoming the villainous enemy of Argo, descended from Ares.