5 “And Jesus, when He had found a young ass, sat thereon.” Here the account is briefly given: for how it all happened may be found at full length in the other evangelists. But there is appended to the circumstance itself a testimony from the prophets, to make it evident that He in whom was fulfilled all they read in Scripture, was entirely misunderstood by the evil-minded rulers of the Jews. Jesus, then, “found a young ass, and sat thereon; as it is written, Fear not, daughter of Zion: behold, your King comes, sitting on an ass's colt.”
Among that people, then, was the daughter of Zion to be found; for Zion is the same as Jerusalem. Among that very people, I say, reprobate and blind as they were, was the daughter of Zion, to whom it was said, “Fear not, daughter of Zion: behold, your King comes, sitting on an ass's colt.” This daughter of Zion, who was thus divinely addressed, was among those sheep that were hearing the Shepherd's voice, and in that multitude which was celebrating the Lord's coming with such religious zeal, and accompanying Him in such warlike array.
To her was it said, “Fear not:” acknowledge Him whom you are now extolling, and give not way to fear when He comes to suffering; for by the shedding of His blood is your guilt to be blotted out, and your life restored. But by the ass's colt, on which no man had ever sat (for so it is found recorded in the other evangelists), we are to understand the Gentile nations which had not received the law of the Lord; by the ass, on the other hand (for both animals were brought to the Lord), that people of His which came of the nation of Israel, and was already so far subdued as to recognize its Master's crib.
Source: Tractates on the Gospel of John (New Advent)