"Rejoice greatly, O Daughter of Zion! Shout, Daughter of Jerusalem! See, your king comes to you, righteous and having salvation, gentle and riding on a donkey, on a colt, the foal of a donkey. I will take away the chariots from Ephraim and the war-horses from Jerusalem, and the battle bow will be broken. He will proclaim peace to the nations. His rule will extend from sea to sea and from the River to the ends of the earth" (Zech. 9:9-10).
Though we are familiar with the prophecy of Zechariah, it is an interesting question to ask if there is any significance to the use of the donkey itself. Why should the king come riding in on a donkey, as opposed to any other animal? Some have said that this is a sign of our Lord's humility - He comes not as a conquering warlord in chariot or upon a battle steed, but upon the most humble of all pack animals to show forth that the nature of His kingship will be built on charity rather than on conquest. Another theological interpretation is that the mule symbolizes human nature, which our Lord weds Himself to through the Incarnation.
These are appropriate interpretations of our Lord's actions on Palm Sunday, but there is another more immediate reason why He chose a donkey, as prophesied by Zechariah. The donkey is, believe it or not, a messianic creature in the Old Testament associated with the son of David. To find the origin of this association, we need to go back to the book of 1 Kings in the days when King David was on the verge of death and Israel was preparing for a succession crisis based on the claims of two rival sons of David.
David has promised the throne to Solomon, his son by Bathsheba. Yet as he drew near to death, we are told that the throne was claimed by another of David's sons, Adonijah, who had offered sacrifices to God and received the fealty of all the important men of the kingdom while excluding Solomon and his supporters. This was told to King David by Nathan the prophet and Bathsheba; when David heard that Adonijah was claiming the kingship and excluding Solomon, he said:
"Call to me Bathsheba." And when she was come in to the king, and stood before him, The king swore, and said: "As the Lord liveth, who hath delivered my soul out of all distress, Even as I swore to thee, by the Lord, the God of Israel, saying: Solomon thy son, shall reign after me, and he shall sit upon my throne in my stead, so will I do this day." And Bathsheba, bowing with her face to the earth, worshiped the king, saying: "May my lord David live for ever." King David also said: "Call me Zadok, the priest, and Nathan, the prophet, and Benaiah, the son of Joiada. And when they were come in before the king, He said to them: Take with you the servants of your lord, and set my son Solomon upon my mule: and bring him to Gihon: And let Zadok, the priest, and Nathan, the prophet, anoint him there king over Israel: and you shall sound the trumpet, and shall say: 'God save king Solomon.' And you shall come up after him, and he shall come, and shall sit upon my throne, and he shall reign in my stead: and I will appoint him to be ruler over Israel, and over Judah" (1 Kings 1:28-35).
We see that the sitting of Solomon upon the mule of David was to be a sign that Solomon was chosen by the king to rule in place of Adonijah, his brother. Perhaps the mule was chosen because only the legitimate son of David would have access to the king's stables; perhaps it was a sign of humility as opposed to Adonijah's pride; we don't really know. The important point is that it was an identification of the true Son of David, a sign to the peoples that David had chosen Solomon. Furthermore, this choice was ratified by a solemn oath by the king himself. When the people saw Solomon seated on the mule of David and anointed by Zadok, they switched their allegiance from Adonijah to Solomon. The king's mule would have been housed in the king's stables, something only David would have had access to. Therefore, to trot Solomon out on the king's mule demonstrates that David has personally selected him as his successor (as opposed to Adonijah, who has proclaimed himself king but does not show any token of succession or intimacy with the king, such as a mule - in 1 Kings 2 Adonijah will try to establish such a connection by asking for one of David's concubines to be his bride, with disastrous consequences for himself).
What is the messianic connection with our Lord? That our Lord is the new Son of David whose kingship over the earth is contested by the Pharisees and those who deny His identity as the Christ. As He prepares to take possession of His kingdom (through the conquest of His Passion), he rides into the holy city on a donkey to identify himself as the true and rightful king and heir to the throne of David, in memory of what David had ordered done for his son Solomon. The fact that David ratified this with an oath in the Old Testament can be seen as God the Father affirming the claims of Jesus, just as He did on the day of our Lord's baptism and at the Transfiguration.
The response of the people of Jerusalem to our Lord's entry make it plain that they, too, understood this Old Testament reference to the triumph of the son of David by their response. In the first place, they cover the ground with palm fronds, something done in the time of the Maccabees to celebrate the triumph over the pagan Greeks and the cleansing of Jerusalem, as we read in 1 Maccabees 13:51:
"And they entered [Jerusalem] on the three and twentieth day of the second month, in the year one hundred and seventy-one, with thanksgiving, and branches of palm trees, and harps, and cymbals, and psalteries, and hymns, and canticles, because the great enemy was destroyed out of Israel."
As they lay palms before Him, they chant Psalm 118 (117), which is one of the most messianic Psalms of the whole Psalter. Psalm 118 begins with the phrase, "Give praise to the Lord, for he is good: for his mercy endureth for ever" and goes on until we get to verse 26, the verse chanted by the crowds, which says, "Blessed be he that cometh in the name of the Lord." Interestingly enough, the very next verse of this Psalm reads:
"We bless you from the LORD'S house. The Lord is God and has given us light. Join in procession with leafy branches up to the horns of the altar" (v. 27) Coincidence?
One other highly interesting fact is that this Psalm 118 also contains a messianic prophecy that is very frequently quoted in the New Testament:
"The stone which the builders rejected; the same is become the head of the corner. This is the Lord's doing , and it is wonderful in our eyes. This is the day which the Lord hath made: let us be glad and rejoice therein" (Ps. 118: 22-24).
The point is that everybody there would have understood Jesus' actions in a very messianic way, based on our Lord's fulfillment of the prophecy of Zechariah, his evoking the memory of David's endorsement of Solomon, the cleansing of Jerusalem during the time of the Maccabees and the messianic prophecies of Psalm 118 regarding the Messiah being a stone which the builders reject. Given this last connection with a stone, isn't it also interesting that our Lord's only recorded comment upon his triumphal entry had to do with stones?
"I say to you that if these shall hold their peace, the stones will cry out" (Luke 21:40).
I'm not sure what the connection is there, but I find it pretty coincidental that our Lord references stones as the people are chanting a Psalm about the cornerstone.
By the way, I know the OT says "mule", but I say close enough since a mule is half-donkey; also, some OT translations call it an "ass" which would make it a donkey. Given the different translations for words between the Hebrew Old Testament, the Septuagint and the New Testament, I think it the theory is still valid despite the OT saying "mule" and the NT saying "donkey."
Well, the point is that there a lot of background to every little statement and action that our Lord says and does and that just a little digging can turn up some pretty amazing connections!