John Chrysostom — as excerpted in the Catena Aurea on Matthew 21:1-9

“To me it seems that He was mounted upon the ass, not only because of the mystery, but to give us a lesson of wisdom, teaching us therein that it needs not to be mounted on horses, but that it is sufficient to employ an ass, and be content with that which is necessary. But enquire of the Jews, what King has entered Jerusalem mounted upon an ass? They can name none other, but this one only.”