Zechariah 9:9
"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."
Messianic Biblical position as written by Shalach Ministries:
Before answering the question as to why we believe that Zechariah 9:9 is Christologicial we must first look at its prophetical references to the New Testament:
Matthew 21:1-11   And when they drew nigh unto Jerusalem, and were come to Bethphage, unto the mount of Olives, then sent Jesus two disciples, Saying unto them, Go into the village over against you, and straightway ye shall find an ass tied, and a colt with her: loose them, and bring them unto me. And if any man say ought unto you, ye shall say, The Lord hath need of them; and straightway he will send them. All this was done, that it might be fulfilled which was spoken by the prophet, saying, Tell ye the daughter of Sion, Behold, thy King cometh unto thee, meek, and sitting upon an ass, and a colt the foal of an ass. And the disciples went, and did as Jesus commanded them, And brought the ass, and the colt, and put on them their clothes, and they set him thereon. And a very great multitude spread their garments in the way; others cut down branches from the trees, and strawed them in the way. And the multitudes that went before, and that followed, cried, saying, Hosanna to the Son of David: Blessed is he that cometh in the name of the Lord; Hosanna in the highest. And when he was come into Jerusalem, all the city was moved, saying, Who is this? And the multitude said, This is Jesus the prophet of Nazareth of Galilee.  Luke 19:29-40  And it came to pass, when he was come nigh to Bethphage and Bethany, at the mount called the mount of Olives, he sent two of his disciples, Saying, Go ye into the village over against you; in the which at your entering ye shall find a colt tied, whereon yet never man sat: loose him, and bring him hither. And if any man ask you, Why do ye loose him? thus shall ye say unto him, Because the Lord hath need of him. And they that were sent went their way, and found even as he had said unto them. And as they were loosing the colt, the owners thereof said unto them, Why loose ye the colt? And they said, The Lord hath need of him. And they brought him to Jesus: and they cast their garments upon the colt, and they set Jesus thereon. And as he went, they spread their clothes in the way. And when he was come nigh, even now at the descent of the mount of Olives, the whole multitude of the disciples began to rejoice and praise God with a loud voice for all the mighty works that they had seen;  Saying, Blessed be the King that cometh in the name of the Lord: peace in heaven, and glory in the highest. And some of the Pharisees from among the multitude said unto him, Master, rebuke thy disciples. And he answered and said unto them, I tell you that, if these should hold their peace, the stones would immediately cry out. 
Mark 11:1-11  And when they came nigh to Jerusalem, unto Bethphage and Bethany, at the mount of Olives, he sendeth forth two of his disciples, And saith unto them, Go your way into the village over against you: and as soon as ye be entered into it, ye shall find a colt tied, whereon never man sat; loose him, and bring him. And if any man say unto you, Why do ye this? say ye that the Lord hath need of him; and straightway he will send him hither. And they went their way, and found the colt tied by the door without in a place where two ways met; and they loose him. And certain of them that stood there said unto them, What do ye, loosing the colt? And they said unto them even as Jesus had commanded: and they let them go. And they brought the colt to Jesus, and cast their garments on him; and he sat upon him. And many spread their garments in the way: and others cut down branches off the trees, and strawed them in the way. And they that went before, and they that followed, cried, saying, Hosanna; Blessed is he that cometh in the name of the Lord: Blessed be the kingdom of our father David, that cometh in the name of the Lord: Hosanna in the highest. And Jesus entered into Jerusalem, and into the temple: and when he had looked round about upon all things, and now the eventide was come, he went out unto Bethany with the twelve. John 12:12-19  On the next day much people that were come to the feast, when they heard that Jesus was coming to Jerusalem, Took branches of palm trees, and went forth to meet him, and cried, Hosanna: Blessed is the King of Israel that cometh in the name of the Lord. And Jesus, when he had found a young ass, sat thereon; as it is written, Fear not, daughter of Sion: behold, thy King cometh, sitting on an ass's colt. These things understood not his disciples at the first: but when Jesus was glorified, then remembered they that these things were written of him, and that they had done these things unto him.
The people therefore that was with him when he called Lazarus out of his grave, and raised him from the dead, bare record. For this cause the people also met him, for that they heard that he had done this miracle. The Pharisees therefore said among themselves, Perceive ye how ye prevail nothing? behold, the world is gone after him. 

 

From the coming of the Grecian conqueror, Zechariah makes a sudden transition, by the prophetical law of suggestion, to the coming of the Jewish Messiah, a very different character emerges.

daughter of Zion:  The theocratic people is called to "rejoice" at the coming of her King, the Messiah (Psalm 2:11).

unto thee:  He (1st. came) (2nd. comes) not for His own gain or pleasure, as earthly kings do, but for the sake of His Church: especially for the Jews' sake, (Romans 11:26).

he is just:  righteous: An attribute constantly attributed to the Messiah (Isaiah
45:21; 53:11; Jeremiah 23:5-6) in connection with salvation.  He does not merely pardon by  ignoring sin, but He justifies by becoming the Lord of righteous fulfillment. Therefore, not merely mercy, but justice, requires the justification of the sinner who by faith becomes one with Yeshua.  God's justice is not set aside by the sinner's salvation, but is magnified and made honorable by it (Isaiah 42:1,21).  His future reign "of righteousness," is also referred to in (Isaiah 32:1).

having salvation:  Not passively, as some interpret it, but "saved," which in context, referring to a "king" coming to reign.  Also, the old versions, the Septuagint, Syriac, and Vulgate, give Saviour.  The Hebrew is reflexive in sense, "showing Himself as a Saviour; . . . having salvation within Himself" for us. Endowed with a salvation which He bestows as a king.  Compare Matthew 1:21, in the Greek, "Himself shall save His people"; that is, not by any other, but by Himself shall He have the ability to save. His "having salvation" for others manifested that He had in Himself that righteousness which was indispensable for the justification of the unrighteous (I Corinthians 1:30; II Corinthians 5:21; I John 2:1). This contrasts beautifully with the haughty Grecian conqueror who came to destroy, whereas the Messiah came to save.  Although, the Messiah shall and will come to take "just" vengeance on His foes, previous to His millennial reign of peace (Matthew 4:1,2).

lowly:  Mild, gentle: corresponding to His "riding on an ass" (not a despised animal, as with us; nor a badge of humiliation, for princes in the East rode on asses, as well as low persons, (Judges 5:10), that is, coming as "Prince of peace"  (Zechariah 9:10; Isaiah 9:6); the "horse," on the contrary is the emblem of war, and shall therefore be "cut off."  Perhaps the Hebrew includes both the "lowliness" of His outward state (which applies to His first coming) and His "meekness of disposition," as Matthew 21:5 quotes it (compare Matthew 11:29), which applies to both His comings.  Both characterize Him with loving sympathy for all men who will hear and obey His Word.  At the same time His coming manifested exaltation (John 5:27; Philippians 2:7-9).

colt:  Untamed, "whereon yet never man sat" (Luke 19:30).  The symbol of a triumphant conqueror and judge (Judges 5:10; 10:4; 12:14).

foal of an ass:  Literally, "asses".   In Hebrew idiom, the indefinite plural for singular (i.e. Geneisi 8:4, "mountains of Ararat," for one of the mountains). The ass (mother of colt) accompanied the colt (Matthew 21:2).  The entry of Jesus into Jerusalem at His first coming is a pledge of the full accomplishment of this prophecy at His second coming.  It shall be "The Day of The Lord" (Psalm 118:24), as at that first Palm Sunday.  The Jews shall then universally (Psalm 118:26) say, what some of them said then, "Blessed is He that cometh in the name of the Lord" (compare Matthew 21:9, with Matthew 23:39); also, "Hosanna," or "Save now, I beseech thee."  "Palms," the emblem of triumph, shall then also be in the hands of His people (compare John 12:13, with Revelation 7:9-10). Then, as on His former entry, shall be the feast of tabernacles (at which they used to draw water from Siloam, quoting Isaiah 12:3). Compare Psalm 118:15, with Zec 14:16.