Duh, because it was on a donkey that Jesus entered Jerusalem. That's the reference, it has nothing to do with rhyming slang at all.

Incidentally the RE teacher at St John Cass Foundation School for Boys in London, has translated one of the gospels into rhyming slang - if there is enough interest he will go on to do more. It sounded pretty good, but there will be people who will object to John the Baptist being referred to as 'Bappy the Prophet guy'.....

LTS