A Greek grave stelé depicting a horse. Notice that it is unrelated to the labours of Heracles - the point is to show how the animal is portrayed in Greek (late classical/hellenistic) art.
This is the story of the man eating horses Diomedes, not to be confused with the Homeric Diomedes, had which Heracles was to capture. As with the other beasts he encountered, these were horrible creatures. The hero travelled to Bistone in Thrace with some companions, sprung a surprise attack and managed to steal the mares. They were however caught up by the locals when they came to the seashore and Heracles himself had to fight the Bistones. The mares was to be checked during the fight and the hero asked the son of Hermes, Abderus to do this. He was unfortunately consumed while the Bistones was defeated; Heracles threw a grave mound for him and founded a city under the friends’ name.
Then he brought the beast with him to Mycenae, only to let them loose as his habit was – they were later killed by wild beasts at Mount Olympus.