The Librarian: The Librarian of the Unseen University is an Orang-Utan. This was not always the case. He was magically transformed by the events chronicled in The Light Fantastic - but since then no member of the University staff can remember who he was beforehand. In addition, a page was torn out of the relevant yearbook; no one knows why, or why the place was marked with a banana skin.(Rumour says the Librarian was Dr. Horace Worblehat, B. Thau, D.M.)
He has resisted all efforts change him back. He must like looking like a rubber-sack-filled-with-water, despite being stronger than one of Ridcully's farts after a virulent dose of WowWow Sauce
From:The Discworld Companion