All this was done, that it might be fulfilled which was spoken by the prophet, saying,

People's New Testament

21:4,5 That it might be fulfilled which was spoken by the prophet. See Isa 62:11 Zec 9:9. The prophet here describes him as riding upon one of the humblest of animals, and in the fulfillment we find, (1) that the animal was borrowed; (2) that he rode without a saddle on borrowed garments; (3) that it was a colt on which no man had ever before rode. Only animals hitherto unused were regarded fit for sacred uses. See Nu 19:2 De 21:3 1Sa 6:7. This is the only instance reported in which the Lord ever rode on any animal.

Scofield Reference Notes

[1] fulfilled

The king's final and official offer of Himself according to Zech 9:9. Acclaimed by an unthinking multitude whose real belief is expressed in Mt 21:11, but with no welcome from the official representatives of the nation, He was soon to hear the multitude shout: "Crucify him."

Matthew Henry's Concise Commentary

21:1-11 This coming of Christ was described by the prophet Zechariah, Zec 9:9. When Christ would appear in his glory, it is in his meekness, not in his majesty, in mercy to work salvation. As meekness and outward poverty were fully seen in Zion's King, and marked his triumphal entrance to Jerusalem, how wrong covetousness, ambition, and the pride of life must be in Zion's citizens! They brought the ass, but Jesus did not use it without the owner's consent. The trappings were such as came to hand. We must not think the clothes on our backs too dear to part with for the service of Christ. The chief priests and the elders afterwards joined with the multitude that abused him upon the cross; but none of them joined the multitude that did him honour. Those that take Christ for their King, must lay their all under his feet. Hosanna signifies, Save now, we beseech thee! Blessed is he that cometh in the name of the Lord! But of how little value is the applause of the people! The changing multitude join the cry of the day, whether it be Hosanna, or Crucify him. Multitudes often seem to approve the gospel, but few become consistent disciples. When Jesus was come into Jerusalem all the city was moved; some perhaps were moved with joy, who waited for the Consolation of Israel; others, of the Pharisees, were moved with envy. So various are the motions in the minds of men upon the approach of Christ's kingdom.