Achilles' stallions were named Balios and Xanthos, born of a union between the harpy Podarge and Zephyros (the West Wind). The horses, who were immortal, were given by Poseidon as a gift to Achilles's father Peleus upon his wedding to the goddess Thetis:
magnificent racing stallions, gifts of the gods to Peleus, shining immortal gifts.
Peleus then gave the horses to Achilles to draw his chariot when his son left home for the Trojan War.
Balios and Xanthos are first mentioned by name in Book 16 of the Iliad, when Patroclus drives them into battle to rally the Achaean troops. Robert Fagles has translated their names as "Dapple" and "Roan Beauty," respectively:
the rapid stallions Roan Beauty [Xanthos] and Dapple [Balios],
the team that raced the gales, magnificent team
the storm-wind filly Lightfoot [Podarge] foaled for the West Wind,
grazing the lush green grass along the Ocean's tides.
The horses' immortal nature makes them difficult to control, and Apollo says to Hector at one point that there is no use running after them, saying:
you're chasing the wild wind—fiery Achilles' team!
They're hard for mortal men to curb and drive,
for all but Achilles—
There is a third horse in Achilles' team, a mortal horse named Pedasos, which Fagles translates as "Bold Dancer." Achilles captured Pedasos in the battle for Cilician Thebe, known here by the epithet "Eetion's city":
And into the traces [Automedon] ran the purebred Bold Dancer [Pedasos]—Achilles seized him once when he stormed Eetion's city,
a mortal war-horse pacing immortal horses now.
The horses are capable of human emotion, and stand weeping over Patroclus's corpse while the Achaeans and Trojans fight for his body:
standing clear of the fray Achilles' horses wept
from the time they first had sensed their driver's death,
brought down in the dust by man-killing Hector.
They are also, briefly, capable of human speech, warning Achilles of his impending doom when, raging with grief for Patroclus, he decides to rejoin the battle:
And Roan Beauty the horse with flashing hoofs
spoke up from under the yoke, bowing his head low
so his full mane came streaming down the yoke-pads,
down along the yoke to sweep the ground . . .
The white-armed goddess Hera gave him voice:
". . . Our team could race with the rush of the West Wind,
the strongest, swiftest blast on earth, men say—still
you are doomed to die by force, Achilles,
cut down by a deathless god and mortal man!"
As Achilles' chariot-team, the horses pull his chariot around the walls of Troy after Achilles has lashed Hector's corpse to it.