Prior to entering Jerusalem, Christ instructed his disciples to acquire for him a donkey (in Matthew's Gospel a donkey and a colt).
Why did Jesus ride an ass? There are four reasons.
First, the prophet Zechariah wrote: "Behold, your king comes to you, triumphant and victorious. He is humble and riding on an ass, on a colt the foal of an ass." (Zech 9:9)
The messianic sign was at once perceived by the crowds who hailed Jesus as their king shouting, "Hosanna! Blessed is he who comes in the name of the Lord!" (Mt 21:9) Catholics still shout this Davidic salutation every time the Holy Sacrifice of the Mass is celebrated. It is an acknowledgment that Jesus is the true Davidic Messiah and king.
Secondly, an ass is integral to the story of Abraham's offering of Isaac, a type of the oblation of the First Born Son as a sign of obedience.
The third reason is that King Solomon Solomon rode to his messianic coronation on a mule that had once belonged to David (1 Kgs 1:33-44).
Fourth, King Jehu rode into Samaria (a kind of false Jerusalem) over the garments of his adherents in order to destroy the temple of the false god Baal (2 Kgs 9:11-10:28). One of the first things Christ does upon entering Jerusalem is bring judgment to the Temple which has become a den of thieves. The typology in this account is rather startling as 2 Kgs chapter nine contains strong messianic language. Like King Jehu, Christ the King comes as a judge over ceremonial regulations.
Look for a post later today on why Christ rode both a donkey and it's foal or colt. Hint: it relates to Judaism and Catholicism.