Known in Omnia as 'The Most Holy St Bobby' and 'Ossory's ass', St Bobby was a donkey that was raised by the Omnian church authorities to bishophood for having traveled through the desert with the Prophet Ossory. Described by Deacon Vorbis as 'highly qualified' for its position in the church aristocracy and by Mustrum Ridcully as 'a righteous ass'. (See Small Gods for further details.)
St Bobby has also been cited by Mr. Slant as a valid legal precedent: if a donkey can become a bishop, then a small dog can legally become principal shareholder and Chairman of the Royal Bank of Ankh-Morpork, which in Slant's opinion requires even less mental acumen than the position of bishop.
Refer to the Roundworld biblical story of Balaam's ass, where the Almighty chides the prophet Balaam whilst having temporarily taken on donkey form. It's there, in The Book of Numbers(Numbers 22:24)... you just haven't read your Bible, have you? I wonder what sort of messianic religion donkeys might create for themselves, based on this tale... ("For God so loved the word that He sent His only beloved donkey...")