Zion's King comes humbly
Matthew says that Jesus' entry into Jerusalem on a donkey fulfilled the prophet's words announcing Zion's humble King.
Rejoice greatly, O Daughter of Zion! Shout in triumph, O Daughter of Jerusalem! See, your King comes to you, righteous and victorious, humble and riding on a donkey, on a colt, the foal of a donkey.
Zechariah 9:9 calls Zion to rejoice because her King comes righteous, victorious, humble, and riding on a donkey; Matthew 21:4-5 quotes the announcement and says that Jesus' entry took place to fulfill it.