Apollodorus Labor 8
In the eighth labor Hercules, with a few companions, heads to the Danube, to the land of the Bistones in Thrace. First, however, he stops off at his old friend Admetus' house. There Admetus tells him the mourning Hercules sees around him is for just some member of the household who has died; not to worry about it. Admetus insinuates the dead woman is no one important, but in this he deceives. It is Admetus' wife, Alcestis,who has died, and not just because it was her time. Alcestis has volunteered to die in place of her husband in accordance with a deal wrangled by Apollo.
Hercules' concern is assuaged by Admetus' statements, so he takes the opportunity to indulge his passions for food, drink, and song, but the staff is appalled by his lighthearted behavior. Finally the truth is revealed, and Hercules, suffering a pang of conscience again, goes off to rectify the situation. He descends into the Underworld, wrestles with Thanatos, and returns with Alcetis is tow.
After a brief scolding of his friend and host Admetus, Hercules continues on his way to an even worse host.
Ares' son Diomedes, King of the Bistones, in Thrace, offers newcomers to his horses for dinner. When Hercules and his friends arrive, the king thinks to feed them to the horses, but Hercules turns the table on the king and after a wrestling match -- prolonged because it is with with the war god's son -- Hercules feeds Diomedes to his own horses. This meal cures the mares of their taste for human.
There are many variations. In some, Hercules kills Diomedes. Sometimes he kills the horses. In one version of Euripides, his Heracles, the hero harnesses the horses to a chariot. The common thread is that the horses eat people and Diomedes dies defending them.
In Apollodorus' version, Hercules brings the horses back to Tiryns where Eurystheus, once again, releases them. They then wander off to Mt. Olympus where wild beasts eat them. Alternately, Hercules breeds them and one of the descendants becomes the horse of Alexander the Great.