Matthew 21:5 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.
This message was prophesied years before Jesus fulfilled it. There is something meek and humble about Jesus riding into town on a new colt. The prophet stated that the King would humbly ride into town on a new colt. Jesus was so pure until he even had to sit on a pure colt. This signified his purity. It is assumed that no one had ridden the new colt before Jesus.
The people are honoring Jesus with a parade. They are shouting Hosanna and the highest praise to the King.
In another scripture, Matthew 21:9, they escort Jesus into Jerusalem:
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.
Jesus came into Jerusalem with honor as being the son of David and King of Sion.
In Mark 11:2, the scripture points out that no one had ever sat on the colt:
And [Jesus] 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.
This scripture points out that no one had ever burdened the colt before. The fact that no one had ever ridden the colt is important. Jesus was so pure until no one could have burdened the animal before Jesus rode it.
The people are proclaiming that Jesus is great and mighty. He had just raised Lazarus from the dead and the people were amazed by him. They knew he was different. They recognized his greatness. Yet Jesus shows up on a new colt or donkey. He would not allow the people to honor him as a great king who would show up on a fine stallion. He was humble and chose a new colt or donkey in keeping with his meek appearance and humble entrance into the city. Jesus was pure of heart just like the new colt.