Blind Io expressly says "we have a monkey-god about the place somewhere", and graciously offers to introduce him to the Librarian, as in his opinion the most suitable deity to hear the petition of one of His followers down there on the Disc. This very nearly causes a moment of diplomatic rift as the Librarian digests the implications of this and appears not to like what he is hearing; the fact that Blind Io is the greatest of the Gods will not help him if he has just implicitly used the M-Word to the librarian. (Io is vulnerable to physical assault, as he discovered when Om head-butted him and broke his nose. As was said at the time, nobody believes in Io all that much - knowing he exists precludes belief. And a Librarian who believes he's just heard the M-word applied to him is operating from a very strong core of belief.) Io is only saved from serious lèse-deity by the intercession of Rincewind, who proposes that a wagon-load of the sort of library stationery supplies Ridcully is too tight to buy will be adequate thanks from the Gods. That, and a red balloon. The monkey-god is never seen: he is probably bright enough to recognise trouble and stay well out of it.
Many Roundworld belief systems have monkey-gods. Hanuman is a deity of the Hindus. Sun-wukong is the Chinese equivalent, or one of them. In Japan he becomes Sungoku. They are thought by scholars to be ultimately reflections of the Hindu God, according to Wiki.
Son Goku is the star of a modern Japanese animation called Dragon Ball and a multimedia Saiyuki character. Sun Wukong seems to have travelled from China to India and taken the form of Hanuman as he was wont to do. Definite confusion there. The Japanese remain divided between Shinto and Buddhism, neither of which they observe very closely and neither of which includes animal gods.
(Abstracted from a discussion on the Talk page of the Gods entry)