The Old Testament points forward to this event. Zechariah 9:9 says, “Rejoice greatly, O daughter of Zion! Shout aloud, O daughter of Jerusalem! Behold, your king is coming to you; righteous and having salvation is he, humble and mounted on a donkey, on a colt, the foal of a donkey.” Mark doesn’t insert this quotation, but Matthew and John do in their record of the triumphal entry. Jesus was clearly fulfilling this prophecy. This is how the king was going to come.
There is also an allusion to this in Genesis 49, where Jacob prophecies of Judah, “The scepter shall not depart from Judah, nor the ruler’s staff from between his feet, until tribute comes to him; and to him shall be the obedience of the peoples. Binding his foal to the vine and his donkey’s colt to the choice vine, he has washed his garments in wine and his vesture in the blood of grapes” (Genesis 49:10-11). The Old Testament pointed to this, even from the very first book. This is the Messiah. He is both Sovereign King and humble Savior.
Jesus has been teaching about the kingdom of God and how it is so different than the kingdom of man. His arrival in Jerusalem in this way is another picture of this. He doesn’t come in on a war horse, decked out in armor, bearing a sword, with defeated captives trailing behind Him. That would have made the experience all the more exciting for the bystanders. But that’s not how Jesus came. He is the sovereign King. He is ruler of all and has power over all. But He comes to Jerusalem in a humble way, riding on a donkey, a donkey that doesn’t even have a saddle. The disciples had to put their own cloaks on it before Jesus sat down.
Riding on a donkey was a proclamation of who Jesus is—the Messiah—in fulfillment of prophecy. And it was also a description of what He is like—humble and peaceful. Like the Zechariah prophecy says, “humble and mounted on a donkey, on a colt, the foal of a donkey.”