Apollodorus on Hercules
Eighth Labor of Hercules (Heracles) - Mares of Diomedes
[2.5.8]The eighth labour he enjoined on him was to bring the mares of Diomedes the Thracian to Mycenae. Now this Diomedes was a son of Ares and Cyrene, and he was king of the Bistones, a very war-like Thracian people, and he owned man-eating mares. So Hercules sailed with a band of volunteers, and having empowered the grooms who were in charge of the mangers, he drove the mares to the sea. When the Bistones in arms came to the rescue, he committed the mares to the guardianship of Abderus, who was a son of Hermes, a native of Opus in Locris, and a minion of Hercules; but the mares killed him by dragging him after them. But Hercules fought against the Bistones, slew Diomedes and compelled the rest to flee. And he founded a city Abdera besides the grave of Abderus who had been done to death, and bringing the mares he gave them to Eurystheus. But Eurystheus let them go, and they came to Mount Olympus, as it is called, and there they were destroyed by the wild beasts.
SOURCE: Loeb Apollodorus, translated by Sir James G. Frazer, 1921.