The Light Fantastic- The Librarian (Discworld) (Wikipedia)
The Librarian appeared in the second novel of the series, The Light Fantastic, where he was transformed into an orang-utan. This had certain advantages for a librarian, such as being able to use his hands and feet for re-shelving books. The other wizards have gradually become used to the situation, to the extent that, from Night Watch: “if someone ever reported that there was an orang-utan in the Library, the wizards would probably go and ask the Librarian if he’d seen it.”