Xeno is one of the Ephebian Philosophers encountered by Brutha and the Omnians during the events of Small Gods, he is described as short and fat. Like his contemporaries, Ibid, Didactylos and others, Xeno is an amalgam of Roundworld counterparts from ancient history - mainly Greek philosophers. He is one of a crowd that disputes in taverns and coins aphorisms, axioms and paradoxes for money. An example is the famous Liar Paradox.
He is also encountered in Pyramids trying to prove the logical argument that an arrow should never hit a moving tortoise.
His interest in paradox and name suggests that he's primarily modelled on Zeno. Not only that, but Xeno's premise that an arrow can never hit a moving tortoise is a parody of two of Roundworld's Zeno's most famous paradoxes - Achilles and the Tortoise (which holds that "in a race, the quickest runner can never overtake the slowest, since the pursuer must first reach the point whence the pursued started, so that the slower must always hold a lead") and The arrow paradox ("if everything when it occupies an equal space is at rest, and if that which is in locomotion is always occupying such a space at any moment, the flying arrow is therefore motionless").