Palm Sunday: Why a donkey?

We look at the reason why Jesus chose to ride a donkey into Jerusalem - as opposed, perhaps, to a horse.  To most of us, donkeys are not the most regal way for a king to ride.  But Jesus came to this planet to create a spiritual kingdom, and looking at Old Testament passages, as well as references to these passages that occur in the New Testament, tell us just why it is that he chose a donkey.