"Go to the village that is in front of you ; as soon as you enter it, you will find an attached donkey, on which no one has ever sat down ; detach him, and bring him ... They bring the donkey to Jesus, put their clothes on the colt, and he sits on it." Mark 11. 2, 7
It is amazing that Jesus chooses a donkey to go up to Jerusalem for the Passover feast during which he will be crucified.
Yet it is the fulfillment of a prophecy of Zechariah : "Rejoice with transports, daughter of Zion ; shout for joy, daughter of Jerusalem! Behold, your king is coming to you; he is righteous and having salvation, humble and riding on a donkey, and on a colt, the cub of a donkey" (Zech 9: 9). The situation can not be clearer for the religious leaders in Jerusalem who meet Jesus in the temple and who are aware of his deeds and actions. The precisions given by the prophet forbid any mistake with regard to Jesus who is the Messiah, the Christ of God. Yet they decide to kill Jesus, and the days of the Lord in the great city are numbered; he will quickly be condemned and crucified.
The choice of the donkey is also a sign of humility and manifests the character of the mission of Jesus. He does not enter Jerusalem on a war horse as the Emperor Vespasian will do in the year 70, when Jerusalem was captured. His purpose is not to drive out the Romans, but to save his people from their sins (Matt 1. 21). It is in the face of justice and humility that Jesus presented himself throughout his ministry and this is how he shows himself in Jerusalem. However, Jesus does not derogate from his title of king, since David mounted such an animal (1 Kings 1. 44).
The choice of a donkey, as Mark specifies it, also discretely emphasizes the sovereignty of the Son of God, Creator of all things. Although uneducated - no one had yet sat on him - the animal allows himself to be gently brought up by Jesus to enter Jerusalem.
Thus, this short information on a donkey highlights the Lord. Object of the message of the prophets, Jesus, in his humility is sovereign, prince of peace, full of sweetness and grace.
Source (Pleasing the Lord)