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 them 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, thus making 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 sent wolves, lions, and bears to kill them.
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