What is a "Jerusalem donkey?"

Most donkeys (except for some breeds, such as the Mammoth Jack, which have had it bred out) have a cross on their backs.

It's a pretty legend that the cross was given to the donkey because he carried Christ to Jerusalem - hence the term "Jerusalem donkey."

The "arm" of the cross is clearly visible on the grey donkey in the foreground. Although the black one likely still has the marking, it is not visible against its much darker coat.