Mt 21:4-9, Mk 11:7-10, Jn 12:14-16
In the Old Testament, the prophet Zechariah prophesied that the coming King, Christ, would enter Jerusalem riding on a colt. (Zec 9:9)
Zec 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.
Jesus entered Jerusalem riding on a colt as prophesied by the prophet Zechariah in the Old Testament. In other words, Jesus is the King of Israel, the Christ. (Matthew 21:4-9, Mark 11:7-10, John 12:14-16)
Matthew 21:4 All this was done, that it might be fulfilled which was spoken by the prophet, saying, 5 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. 6 And the disciples went, and did as Jesus commanded them, 7 And brought the ass, and the colt, and put on them their clothes, and they set him thereon. 8 And a very great multitude spread their garments in the way; others cut down branches from the trees, and strawed them in the way. 9 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.
Mark 11:7 And they brought the colt to Jesus, and cast their garments on him; and he sat upon him. 8 And many spread their garments in the way: and others cut down branches off the trees, and strawed them in the way. 9 And they that went before, and they that followed, cried, saying, Hosanna; Blessed is he that cometh in the name of the Lord: 10 Blessed be the kingdom of our father David, that cometh in the name of the Lord: Hosanna in the highest.
John 12:14 And Jesus, when he had found a young ass, sat thereon; as it is written, 15 Fear not, daughter of Sion: behold, thy King cometh, sitting on an ass's colt. 16 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.