He prepares the entry (Mark 11:2–3) just like King David prepared the entry of his son Solomon (1 Kings 1:33). Jesus is the king, thus he rides on a young animal. It is not an animal used before; the first ride is made by the king. As king he claims the use of the colt, and people obey. He fulfills the prophesy of Zechariah 9:9: 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 does not mention this prophecy, but we read it in Matthew 21:5 and John 12:15. So Jesus enters as a king, the Son of David (Mark 10:47–48). He directs the fulfillment of Zechariah 9. People do not stand still and think about this prophesy (John 12:16). In the time of Jesus kings and emperors often made their entry proudly into their capital town on a horse or a chariot as a sign of battle and siege, but Jesus comes on a donkey, in peace and humility.
2 en leur disant: Allez au village qui est devant vous; dès que vous y serez entrés, vous trouverez un ânon attaché, sur lequel aucun homme ne s'est encore assis; détachez-le, et amenez-le.