After seven failed attempts, King Eurystheus has still not rid himself of his cousin Heracles. Perhaps, he thinks, combat with a warrior-king of divine birth, some man-eating mares, and a savage tribe would to be enough; and certainly, things do not look good for our hero at first.
ARES, the god of war,* had a son named Diomedes, lord of the Bistones, a warrior-tribe that lived near Lake Vistonida in Thrace.* Down by the sea Diomedes kept a string of savage mares, chained to bronze mangers in which he gave them man’s flesh to eat.
Understandably, in sending Heracles to steal these man-eating mares from their warlike master and his barbarous tribe, King Eurystheus hoped that even his cousin would meet his match.
Heracles began by driving the mares into the sea, but to his horror they dragged his friend Abderus in after them, who drowned while Heracles fended off the enraged Bistones. In a fury of vengeance he fed Diomedes to his own mares, which instantly became tame, and followed Heracles back meekly to Tiryns.
Having no real use for them, Eurystheus let the placid mares wander onto Mount Olympus, where wild beasts devoured them, though rumour had it that one of their descendants became a favourite of Alexander the Great.
* Ares in Greek mythology corresponds roughly to Mars in Roman mythology; but for the Greeks, Ares was very much the god of brute force and violence only; the deity of military strategy and generalship was his sister Athena.
* See Google Maps.