The Concordia Journal observes that John is using the donkey as key in this text. The donkey pointed to the Passion Week. The donkey, though used as kingly transportation in the Solomonic days, was also a symbol of peace.
He was riding on a donkey to show just how he would defeat all our enemies.
Jesus does not come to war against His enemies with human weapons, but with humility and blood.
Resurrectio et Vita 
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