The Uberwald Incident
The Librarian is a faculty member, and so he must have once been a human wizard. A strong wave of magic, altering reality, transformed the Librarian into an orang-utan. After things settled down, the Librarian decided he liked being an orang-utan, and fought all efforts to transform him back to human. The Librarian found out that opposable toes come in very handy when climbing up the book-shelves and sorting books. Also, a very large and strong orang-utan has more plausible arguments when talking about dog-ears and overrun return deadlines.
In appearance he has the rubber-sack-filled-with-water look of a well grown, 300lb male. Very soon, people forgot how the Librarian looked as a human, or what his given name was (though the latter is true for most other faculty members as well). This was helped along by the Librarian methodically removing, defacing or otherwise altering any and all photographs and records of his name in the University files. Nowadays, only people very new to the University or Ankh-Morpork will find themselves surprised that there is an orang-utan in a University Library, or in a pub. Indeed, if someone were to point out that there is an orang-utan in the library, the wizards would probably ask the librarian if he had seen it. All the Librarian ever utters is ‘oook’ or ‘eeek’, but by now the wizards are so used to this that they understand the Librarian perfectly.
Despite his usually easy going nature, it should be noted that the Librarian is an ape rather than a monkey and is extremely eager to correct people on the subject in uncharacteristically violent ways. He can also become physically assertive when enforcing the prohibition against open flame inside the University’s library, or when otherwise acting in defense of his literary charges.