This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Xanthus (Greek)
In Greek mythology, Xanthus was the name of Achilles horse, the offspring of Zephyrus, the west wind. Xanthus was given the power of speech by the gods in order to tell Achilles who had killed his friend Patroclus. Balius and Xanthus were, according to Greek mythology, two immortal horses, the offspring of the harpy, Podarge and the West wind, Zephyrus.
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. When Xanthus was rebuked by the grieving Achilles for allowing Patroclus to be slain, Hera granted Xanthus human speech allowing the horse to say that a god had killed Patroclus and that a god would soon kill Achilles too. After this, the Erinyes struck the horse dumb.