Heracles' eighth labor was to steal the Mares of Diomedes. The Mares of Diomedes were four uncontainable - and man-eating - horses that were owned by King Diomedes of Thrace. The horses were chained to a bronze manger in order to keep them somewhat under control.
Heracles traveled to Thrace so that he could capture the beastly horses. In order to avoid trouble with Diomedes (in the form of a knife to his throat), Heracles made sure to stay awake, and he cut the horses free of their chains in the dark of night. He spooked the mares so that they would run, and they fled to a high area of ground on a peninsula. Heracles cut a trench through the peninsula, and filled it with water. This made the land an island, and the horses were trapped.
With the horses temporarily secured, Heracles waited for Diomedes' inevitable arrival. When he arrived, Heracles killed the king swiftly using an axe, and then fed him to his own horses.
Full bellies calmed the horses enough for Heracles to bind their mouths close, and take them to Eurystheus, who dedicated the horses to Hera. Depending on the version of the myth that you read, Eurystheus either sent the mares to Mount Olympus for sacrifice (Zeus refused them and sent beasts to slaughter them), or they were set loose to roam Argos in their newly calmed state.
