This king made the Mares of Diomedes eat man's flesh. This was than one of Herakles's 12 labors, the 8th one. To get them not to eat him he took King Diomedes and made the Mares eat the King. Then the horses tame and never eat flesh again.
Herakles wrestling with Diomedes (whom Herakles would later feed to the king's own mares)
Herakles having defeated Diomedes about to feed them to Diomedes Mares