Your king comes on a donkey
Whether the prophecy's king comes in peace or judgment, and why Matthew names two animals.
Zechariah 9:9-10
9Rejoice greatly, daughter of Zion! Shout, daughter of Jerusalem! Behold, your King comes to you! He is righteous, and having salvation; lowly, and riding on a donkey, even on a colt, the foal of a donkey.
10I will cut off the chariot from Ephraim, and the horse from Jerusalem; and the battle bow will be cut off; and he will speak peace to the nations: and his dominion will be from sea to sea, and from the River to the ends of the earth.
Matthew 21:1-11
1When they came near to Jerusalem, and came to Bethsphage, to the Mount of Olives, then Jesus sent two disciples,
2saying to them, “Go into the village that is opposite you, and immediately you will find a donkey tied, and a colt with her. Untie them, and bring them to me.
3If anyone says anything to you, you shall say, ‘The Lord needs them,’ and immediately he will send them.”
4All this was done, that it might be fulfilled which was spoken through the prophet, saying,
5“Tell the daughter of Zion, behold, your King comes to you, humble, and riding on a donkey, on a colt, the foal of a donkey.”
6The disciples went, and did just as Jesus commanded them,
7and brought the donkey and the colt, and laid their clothes on them; and he sat on them.
8A very great multitude spread their clothes on the road. Others cut branches from the trees, and spread them on the road.
9The multitudes who went in front of him, and those who followed, kept shouting, “Hosanna to the son of David! Blessed is he who comes in the name of the Lord! Hosanna in the highest!”
10When he had come into Jerusalem, all the city was stirred up, saying, “Who is this?”
11The multitudes said, “This is the prophet, Jesus, from Nazareth of Galilee.”
What each tradition says
Rabbinic / Midrashic
Rashi says Zechariah's words "Behold! Your king shall come to you" can only be interpreted as referring to the King Messiah. He supports this by the statement "and his rule shall be from sea to sea," noting that Israel had no such ruler during the days of the Second Temple.
Rashi (R. Solomon ben Isaac), Rashi on Zechariah 9:9 — The Judaica Press Tanach with Rashi, A. J. Rosenberg trans., served by Sefaria — Commentary on Zechariah 9:9
Patristic
Augustine says there is no real antagonism when both objects may have been included in the occurrence, even if one writer specifies only one thing and another only the other. Still less is there contradiction when one writer particularizes one object and another instances both.
Augustine, NPNF1-06. St. Augustine: Sermon on the Mount; Harmony of the Gospels; Homilies on the Gospels — Harmony of the Gospels, Book II, ch. 66 (§127)
Catholic
Haydock says the Jews, if not wilfully blinded, could not mistake the Messiah, who is described as both humble and noble. Unable to reconcile these qualities, some imagined two Messiahs, one glorious and one poor; others accepted one but rejected either the abject or exalted things spoken of him. Only the Church can reconcile these apparent contradictions in Christ's character.
George Leo Haydock, George Haydock's Catholic Bible Commentary — Commentary on Zechariah 9:9
Orthodox
Chrysostom says the colt signifies the church, the new people, which was once unclean but became clean after Jesus sat on them. He sees the image preserved throughout: the disciples loose the asses, for by the apostles both they and we were called and brought near.
John Chrysostom, NPNF1-10. St. Chrysostom: Homilies on the Gospel of Saint Matthew — Homily LXVI (on Matthew 21:1-11)
Reformed / Protestant
Calvin says the prophet briefly shows how the Church would be restored: a king from the tribe and family of David would arise again to restore all things to their ancient state. He adds that this is the view given everywhere by the Prophets, since the hope of the ancient people, like our hope, was founded on Christ.
John Calvin, Commentary on Zechariah, Malachi (John Owen trans.) — On Zechariah 9:9
Evangelical
John Wesley identifies the promised king as the Messiah, the righteous one who comes to fulfill all righteousness. He has salvation to bestow on all who believe in him.
John Wesley, Explanatory Notes upon the Old Testament — Note on Zechariah 9:9