Theutus is a demon described by the Renaissance magician Cornelius Agrippa.
According to Cornelius Agrippa Theutus belongs to the order of demons known as the Vessels of Iniquity, also called the Vessels of Wrath: "these are the inventors of evil things and of all wicked arts, as in Plato, that devil Theutus" (Occult Philosophy, Book 3, Chapter 18). Agrippa placed the Vessels of Iniquity in Binah, the third Sephirah on the Tree of the Kabbalah.
Agrippa's reference is to Plato's dialogue Phaedrus where the Egyptian god of magic and learning, Thoth, is described: "The story is that in the region of Naucratis in Egypt there dwelt one of the old gods of the country, the god to whom the bird called Ibis is sacred, his own name being Theuth. He it was that invented number and calculation, geometry and astronomy, not to speak of draughts and dice, and above all writing."
Theutus is a degenerate version of the great Egyptian god Thoth. Since Thoth was credited with teaching humanity numbers and their manipulation, he is naturally the god of gambling, which requires the use of numbers.