Arion (mythology), Hippocampus (mythology), Mares of Diomedes
Balius and xanthus
Balius (/ˈbeɪliəs/; Βάλιος, Balios, possibly "dappled") and Xanthus (/ˈzænθəs/; Ξάνθος, Xanthos, "blonde") were, according to Greek mythology, two immortal horses, the offspring of the harpy Podarge and the West wind, Zephyrus; following another tradition, their father was Zeus.
Note: Balius and Xanthus are the Latin forms of the Greek names Balios and Xanthos.
Horses of Achilles
Poseidon gave the two horses to King Peleus of Phtia, as a wedding gift, when Peleus married the Ocean goddess Thetis. Peleus later gave the horses to his son Achilles who took them to draw his chariot during the Trojan War.
Book 16 of the Iliad tells us that Achilles had a third horse, Pedasos (maybe "Jumper", maybe "Captive"), which was yoked as a "trace horse", along with Xanthus and Balios. Achilles had captured Pedasos when he took the city of Eetion. Pedasos was mortal, but he could keep up with the divine horses. Sarpedon, prince of Lycia and ally of Troy, killed Pedasos when his spear missed Patroclus. Achilles' comrade-in-arms Patroclus used to feed and groom these horses. In the Iliad, it is told how, when Patroclus was killed in battle, Xanthus and Balius stood motionless on the field of battle, and wept.
At Iliad 17.474-8, Automedon, Achilles' charioteer, states that only Patroclus was able to fully control these horses. When Xanthus was rebuked by the grieving Achilles for allowing Patroclus to be slain, Hera granted Xanthus human speech which broke Divine law, allowing the horse to say that a god had killed Patroclus and that a god would soon kill Achilles too. After this, the Erinyes struck the horse dumb.
Horse of Diomedes
Another Xanthus, not to be confused with the horse mentioned above, was one of the horses of Diomedes of Thrace, who fed these animals on human flesh. The capture of these horses was the eighth of the Twelve Labors of the Great Heracles.