10 But who is this that comes down? He says in what follows, And with the sun He endures, and before the moon generations of generations. And again another of the Prophets says, Rejoice greatly, O daughter of Sion, shout, O daughter of Jerusalem. Behold! Your King comes unto you, just and having salvation. Kings are many; of which do you speak, O Prophet? Give us a sign which other Kings have not. If you say, A king clad in purple, the dignity of the apparel has been anticipated. If you say, Guarded by spear-men, and sitting in a golden chariot, this also has been anticipated by others. Give us a sign peculiar to the King whose coming you announce. And the Prophet makes answer and says, Behold! Your King comes unto you, just, and having salvation: He is meek, and riding upon an ass and a young foal, not on a chariot. You have a unique sign of the King who came. Jesus alone of kings sat upon an unyoked foal, entering into Jerusalem with acclamations as a king. And when this King has come, what does He? Thou also by the blood of the covenant hast sent forth your prisoners out of the pit wherein is no water.
Source: Catechetical Lectures (New Advent)