thumb|Balius and Xanthus
Balius (; Ancient Greek: Βάλιος, Balios, possibly "dappled") and Xanthus (; Ancient Greek: Ξάνθος, Xanthos, "blonde") were, according to Greek mythology, two immortal horses, the offspring of the harpy Podarge and the West wind, Zephyrus. In other traditions, Poseidon is the father of Xanthus along with another horse named Cyllarus to an unnamed mother. It is possible that Xanthus's ability to speak prophetically may be related to Arion, another mythical horse reported to have saved Adrastus from the war of the Seven against Thebes with his prophetic abilities in Statius's Thebaid. In lines 17.474–17.486, the Myrmidon chief Alcimedon appears and takes the reins at Automedon's request. In lines 19.400–19.418, when the horses were rebuked by the grieving Achilles for allowing Patroclus to lie fallen on the battlefield, instead of bringing him back to the Greeks, Hera granted Xanthus human speech, allowing the horse to say that a god, Apollo, had killed Patroclus, and that a god and a mortal would soon kill Achilles too. After this, the Erinyes struck the horse dumb.
Based on fragments from Alcman and Stesichorus, an alternative story of the horses can be derived.