Rejoice greatly, O daughter of Sion, shout for joy, O daughter of Jerusalem: BEHOLD THY KING will come to thee, the just and saviour: he is poor, and riding upon an ass, and upon a colt the foal of an ass.  And I will destroy the chariot out of Ephraim, and the horse out of Jerusalem, and the bow for war shall be broken: and he shall speak peace to the Gentiles, and his power shall be from sea to sea, and from the rivers even to the end of the earth.
Saint Clement of Alexandria wrote about this passage:
Rejoice greatly, O daughter of Sion; tell aloud, O daughter of Jerusalem: behold, your King comes, just, meek, and bringing salvation; meek truly is He, and riding on a beast of burden, and a young colt. Zechariah 9:9; Genesis 49:11 It was not enough to have said colt alone, but He added to it also young, to show the youth of humanity in Christ, and the eternity of simplicity, which shall know no old age. And we who are little ones being such colts, are reared up by our divine colt-tamer. But if the new man in Scripture is represented by the ass, this ass is also a colt.
And he bound, it is said,
the colt to the vine, having bound this simple and childlike people to the word, whom He figuratively represents as a vine. For the vine produces wine, as the Word produces blood, and both drink for health to men— wine for the body, blood for the spirit.