Diomedes was a cruel chief who lived in a wild part of the land, next to the Black Sea. He kept 4 horses who, because they were fed the human flesh of strangers that Diomedes killed, were as wild as their owner. Each horse had a name: Podagros (‘the fast’); Lampon (‘the shining’); Xanthos (‘the blond’); and Deinos (‘the terrible’). Heracles’s next labor was to steal these mares.
What Heracles did not know until he arrived was that Diomedes controlled these horses by tying them with bronze chains to a manger, where they were fed the flesh.
There are different versions of the story as to how Heracles captured the horses. In the simplest one, Heracles simply captures the king and throws him in the manger, where the horses eat him. No longer having their wicked master, the horses become calm
and Heracles leads them safely back to King Eurystheus.
Another version is that Heracles brings a group of people with him to help. While Heracles goes to fight Diomedes and his men, a young boy named Abderus is left to watch the horses. Unfortunately, Abderus is then eaten by the horses. When Heracles sees what has happened he is so angry he feeds Diomedes to the horses too, after which they become calm. To remember the dead Abderus, Heracles founds the city of Abdera.
There is, however, a third version. In this story Heracles again brings a group of men with him, including the young man Abderus. Heracles cuts the chains that are keeping the horses tied, and then scares them onto the higher ground of a peninsula. He then digs a trench and fills it with water, leaving the horses stuck on an island.
Diomedes, seeing that his horses have gone, chases Heracles. However, when he arrives Heracles quickly kills him and then feeds the body to the horses. While they are eating Heracles then ties their mouths, keeping himself safe.
Although each story is slightly different, the common thread is that Diomedes is fed to his own horses, which then keeps them calm. Heracles then took the horses back to Eurystheus.
What happened to the horses after they are given to Eurystheus also has different versions. In one, they are allowed to wander around Argos. In a second, more bloody version, however, they are offered to Zeus, the king of the Gods, but he rejects them, and has wild animals eat the horses.