Rejoice greatly, O daughter of Zion; shout, O daughter of Jerusalem: behold, thy King cometh unto thee: he is just, and having salvation; lowly, and riding upon an ass, and upon a colt the foal of an ass. Rejoice greatly, O daughter of Zion - See this prophecy explained on Matthew 21:5 (note).
Behold, thy King cometh - Not Zerubbabel, for he was never king; nor have they had a king, except Jesus the Christ, from the days of Zedekiah to the present time.
He is just - The righteous One, and the Fountain of righteousness.
Having salvation - He alone can save from sin, Satan, death, and hell.
Lowly - Without worldly pomp or splendor; for neither his kingdom, nor that of his followers, is of this world.
Riding upon an ass - God had commanded the kings of Israel not to multiply horses. The kings who broke this command were miserable themselves, and scourgers to their people. Jesus came to fulfill the law. Had he in his title of king rode upon a horse, it would have been a breach of a positive command of God; therefore, he rode upon an ass, and thus fulfilled the prophecy, and kept the precept unbroken. Hence it is immediately added: -