Prior to entering Jerusalem, Christ instructed his disciples to acquire for him a donkey (in Matthew's Gospel and donkey and a colt).
Why is this?
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.