Achilles’ immortal horse that could speak was Xanthus. In Homer’s Iliad, Xanthus is one of the two divine horses—along with Balius—that drew Achilles’ chariot during the Trojan War.
The horses were associated with extraordinary parentage and were given to Peleus, Achilles’ father, before passing to Achilles. A third horse, Pedasus, joined the team but was mortal. After Patroclus died, Achilles blamed the horses for failing to bring him safely from battle. Hera then granted Xanthus human speech.
Xanthus revealed that Apollo had caused Patroclus’s death and ominously foretold that a god and a mortal would soon kill Achilles. The Erinyes, divine agents of retribution, immediately silenced him. Balius is a frequent mix-up because he was also immortal and belonged to Achilles, but Xanthus is the horse specifically described as speaking in the Iliad.