To repeat the Gospel texts:
Go into the village over against you, and straightway ye shall find an ass tied, and a colt with her: loose them, and bring them unto me.
And if any man say ought unto you, ye shall say, The Lord hath need of
them; and straightway he will send them. All this was done, that it
might be fulfilled which was spoken by the prophet, saying, Tell ye
the daughter of Sion, Behold, thy King cometh unto thee, meek, and
sitting upon an ass, and a colt the foal of an ass (Matthew 21:2-5).
And Jesus, when he had found a young ass, sat thereon; as it is written, Fear not, daughter of Sion: behold, thy King cometh, sitting
on an ass’s colt (John 12:14-15)
Beyond humility and the fulfillment of prophesy, some Church Fathers saw certain symbolism in the donkey and/or colt themselves. These views are summarized in the Greek commentaries of Theophylact of Ohrid (1050-1107). In his commentaries, the donkey and its colt are seen to be symbols of the Jews and the Gentiles - the Jews tamed already, so to speak, and the Gentiles yet to be so:
He fulfills the prophesy1 both literally, and in a spiritual sense. He fulfills it literally by sitting as He did in view of all. He fulfills it in a spiritual sense by sitting upon the ass, the burdened Jews, and also upon the foal, the Gentiles, who were coltish, untamed and unruly.2. For the ass and the colt had been tethered by the reins of their own sins. Two were sent to loose them, Paul to the Gentiles, and Peter to the circumcised, that is, to the Jews. And even now, there are two that loose us from our sins, the Epistles and the Gospel.3
His sitting upon an ass symbolizes how God's dispensation of grace would spread beyond the confines of Israel. The donkey, belonging to the category of unclean animals according to the law, represents the defiled Gentiles. Jesus, the Word of God, subdued (as one does a cold) this new, unruly, and uninstructed race. Once it was tamed and had submitted to Him, He led it into the true Jerusalem. For the Lord indeed gathered the Gentiles into heaven, after they became His people and had accepted His preaching.4
These interpretations are consistent, I think, with what follows in Zechariah. The phrase translated as "Gentiles" in the New Testament is the Greek ἔθνη (ethnē), which means "peoples" or "nations". It is exactly the phrase that appears in the Septuagint version of Zechariah 10:10 - the verse following the prophesy alluded to by Matthew and John:
καὶ ἐξολεθρεύσει ἅρματα ἐξ Εφραιμ
καὶ ἵππον ἐξ Ιερουσαλημ
ἐξολεθρευθήσεται τόξον πολεμικόν
καὶ πλῆθος καὶ εἰρήνη ἐξ ἐθνῶν
καὶ κατάρξει ὑδάτων ἕως θαλάσσης
καὶ ποταμῶν διεκβολὰς γῆς.
And he shall destroy the chariots out of Ephraim,
and the horse out of Jerusalem,
and the bow of war shall be utterly destroyed;
shall be abundance and peace out of the nations;
and he shall rule
over the waters as far as the sea,
and the rivers to the ends of the
1. Rejoice greatly, O daughter of Sion; proclaim it aloud, O daughter of Jerusalem; behold, the King is coming to thee, just, and a Saviour; he is meek and riding on an ass, and a young foal (Zechariah 9:9 LXX, Brenton). The Masoretic Text departs a little from the Septuagint, indicating "having salvation" instead of "Saviour".
2. The sceptre shall not depart from Judah, Nor a lawgiver from between his feet, Until Shiloh come; And unto him shall the gathering of the people be. Binding his foal unto the vine, And his ass’s colt unto the choice vine; He washed his garments in wine, And his clothes in the blood of grapes (Genesis 49:10-11).
3. The Explanation of the Holy Gospel According to St. Matthew (tr. from Greek; Chrysostom Press, 2008), p.175
4. The Explanation of the Holy Gospel According to St. John (tr. from Greek; Chrysostom Press, 2007), p.196