In Greek mythology donkey is identified with Dionysus. There is also a story of King Midas who got the ears of donkey because of a wrong judgment during the musical contest. He preferred Pan instead of Apollo which means that he chose his sensuality instead of the taste of harmony.
The Christian symbolism is quite ambivalent. Donkey has a significant role in the Bible. Joseph and Mary were riding a donkey on their way to Egypt. On the Palm Sunday Christ came to Jerusalem on the back of a donkey. It is very important to note that the donkey was female. The symbolism of a female donkey is absolutely different. The Satanic donkey is opposed to a female donkey of knowledge, symbol of humility, poverty, courage and peace. This is quite a paradox because the other explanations tell us that, in fact, at that time to ride on a donkey was seen as a sign of richness befitting the House of David. Usually people were going on foot. This primal significance was changed later because of the usage of horses. So, the later symbolism of Christ’s poverty and humility when riding a donkey was a pure adaptation to the current cultural circumstances and understanding.