Why a donkey?

Both the sermon today and our Sunday school lesson mentioned Jesus riding on a donkey into Jerusalem. 

One of the points of the sermon was that Jesus rode a donkey as an act of humility.  In Sunday school we talked about how Jesus riding the donkey was a fulfillment of prophecy in Zechariah.

I would like to explore the idea more than this.  I just wonder if the humility idea is our own impressions of what it means to ride on a donkey.  Did the people in Jerusalem during that time see it as an act of humility?

One reference I found this evening said that the donkey was an animal of peace; by riding a donkey Jesus was saying that he came in peace.  A horse is an animal of war.

In 1 Kings 1, when King David is close to death and is naming his successor, he tells a priest and Nathan the prophet, to find Solomon and to have him ride David's mule into the city.  Solomon is then anointed as king.  So, could riding the donkey in Jerusalem have something to do with his connection to King David?  Ascension to the throne of David?

Abraham and Isaac rode on a donkey close to the location where Abraham had been instructed to sacrifice Issac.  Could there be a link to that event?  To the sacrifice of a son?

Moses' placed his wife and children on a donkey.  A judge had 30 of his sons ride 30 donkeys.  Is it just that donkeys are popular means of transportation?

I would like to more about the donkey (above and beyond our normal assumptions).