In Greek mythology, Diomedes, owner of the man-eating mares, was king of Thrace.
The mares belonged to Diomedes, a violent king of the Bistones, a Thracian people. Their diet of human flesh made them exceptionally dangerous, and Eurystheus ordered Hercules to capture them as his eighth labor.
Hercules brought the animals under control in different versions of the myth. A common account says that he fed Diomedes to his own mares, after which their nature changed and they could be handled. Hercules then took them back to Eurystheus.
This Diomedes should not be confused with the Greek warrior Diomedes who fought in the Trojan War. They are different mythological figures. The Thracian king is identified by his connection with the mares and with the Bistones, while the Trojan War hero was associated with Argos and the Achaean army.