PHLOGEUS, HARPAGOS, XANTHOS and KYLLAROS were four immortal horses bestowed upon the Dioskouroi twins by the gods Hermes and Hera. They carried the twins both during their mortal years and after their elevation to godhood.

"The horses given to Hera by Poseidon are called Kyllaros and Xanthos by the lyric poet Alkman: Kyllaros was said to have been given to Polydeukes, Xanthos to his brother." - Greek Lyric II Alcman, Frag 25 (from Scholiast on Virgil, Georgics)

"Stesichorus in his Funeral Games of Pelias says that Hermes gave the Dioskouri [who were competing in the chariot-race] Phlogeus and Harpagos, swift foals of [the Harpyiai] Podarge, while Hera gave them Xanthos and Kyllaros." - Greek Lyric III Stesichorus, Frag 178 (from Etymologicum Magnum)

H28.1A DIOSKOUROS

H28.1B DIOSKOUROS

H28.2 DIOSKOUROI

"Such [of the finest horses] was Cyllarus, tamed by the reins of Amyclaean Pollux, and those whose fame Greek poets recount, the two steeds of Mars [Ares], and the pair of the great Achilles.- Virgil, Georgics 3.89

"Shouldst thou be pleased to ride a horn-footed horse, with hand more agile on the rein than Castor’s thou couldst guide the Spartan Cyllarus." - Seneca, Phaedra 810

"[Kastor] left [his horse] Cyllarus to fatten upon the grass of Amycle [when he joined the Argonauts." - Valerius Flaccus, Argonautica 1.426

"Amphiaraus, next favourite for the prize [in the chariot race at the first of the Nemean Games], aloft in his chariot drives Oebalian steeds; thy progeny, Cyllarus, stealthily begotten while far away by the mouth of Scythian Pontus Castor was exchanging for the oar the Amyclean rein. Snow-white his own raiment, snow-white are the coursers that lend their necks to the yoke, his [Amphiaraus’] helm and fillet [worn by a seer] match the whiteness of his crested plume ...
[In the race] neither goads nor lashes now suffice, but with shout of name does ... the Danaan augur [Amphiaraus] chide fleet Aschetos and Cygnus well so-called …
[Apollon] lends strength and refreshment [to the horses of Amphiaraus]; swifter than the East wind he flies, as though the barrier were just fallen and he were starting on the race, and calling aloud on nimble Caerus and snow-white Cygnus, plies their necks with blows and shakes the reins upon their backs [and won the race]." - Statius, Thebaid 6.328

Sources:

• Greek Lyric III, Stesichorus - Greek Lyric C7th-6th BC
• Virgil, Georgics - Latin Idyllic C1st BC
• Seneca, Phaedra - Latin Tragedy C1st AD
• Valerius Flaccus, The Argonautica - Latin Epic C1st AD
• Statius, Thebaid - Latin Epic C1st AD