Troias Tragōidia

Tempestuous Immortal Chariot. The Noble Phantasm of Rider of “Red,” Achilles. An A rank chariot driven by three steeds. The horses consist of the two divine horses which the sea god Poseidon gave to Achilles' father Peleus as a wedding gift, Xanthus and Balius, and the famous horse which Achilles stole from a city he attacked using his divine horses, Pedasos. According to legend, Achilles' chariot is thought to been driven by three or four horses. Xanthus and Balius apparently served as the central pivot while Pedasos had a supporting role.
As divine horses received from Poseidon, Xanthus and Balius were clearly stated to have both been immortal in the original texts. After being summoned as a Servant's Noble Phantasm, they aren't quite immortal, but at the very least, they can be considered as tough as Servants themselves.
When Achilles is summoned as a Rider, his most harshly mana-consuming Noble Phantasm is this chariot. In the worst case, it is estimated that it can consume enough mana to summon another Servant besides Achilles.
Xanthus is a truly unpleasant horse who has fun telling Achilles when he falls into a dangerous situation.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia