What man-eating horses did Heracles steal as his eighth labor?

The man-eating horses stolen by Heracles as his eighth labor were the Mares of Diomedes.

These wild, dangerous horses belonged to Diomedes, the king of the Bistones in Thrace. Their unnatural appetite for human flesh made them uncontrollable, and some retellings add that they breathed fire. They were kept tethered with chains, often described as being secured to a bronze manger in the city of Tirida.

Heracles and his companions seized the herd and drove it toward the sea. In one famous version, Heracles left the mares with his companion Abderus while he confronted Diomedes. The horses killed Abderus, so Heracles fed Diomedes to the animals; after consuming their master, they became calm enough to transport.

The mares are sometimes called the Mares of Thrace, which can make the answer seem less exact, but both names refer to the same herd. They should not be confused with Hippocamps, the sea-horses associated with Poseidon, or the mortal Diomedes who fought in the Trojan War.