A colt is the foal of a donkey. Jesus rode into Jerusalem on the back of a borrowed donkey’s colt, one that had never been ridden before. This is explained in Matthew 21:1-5:
“As they approached Jerusalem and came to Bethphage, on the Mount of Olives, Jesus sent two disciples, saying to them, “Go to the village ahead of you, and at once you will find a donkey tied there, with her colt by her. Untie them and bring them to me. If anyone says anything to you, tell him that the Lord needs them, and he will send them right away. This took place to fulfil what was spoken through the prophet, saying, “Say to the daughter of Zion, ‘See, your king comes to you, gentle and riding on a donkey, on a colt, the foal of a donkey.’ ”
Matthew quotes from the prophecy of Zechariah 9:9, applying it to Jesus: “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.”
Why a colt and not a powerful horse? Well, Bible history tells us that leaders rode horses if they rode to war, but donkeys if they came in peace. First Kings 1:33 mentions Solomon riding a donkey on the day he was recognized as the new king of Israel. King David gave him is own donkey to show he had been appointed by David as the new King.
Other instances of leaders (the judges of Israel) riding donkeys are Judges 5:10; 10:4; 12:14; and 2 Samuel 16:2. Of greater significance is the mention of a donkey in Zechariah 9:9-10 which fits the description of a king who would be “righteous and having salvation, gentle.” Rather than riding to conquer, this king would enter in peace. In Israel, riding into a city on a donkey was a sign of a ruler coming in peace.
That is why Jesus rode into Jerusalem on the back of a colt, to fulfil the prophecy that the Messiah would come into Jerusalem in peace, upon the back of a colt, the foal of a donkey.
The article in the link below says this:
“The horse has usually symbolized times of war, but the donkey, times of peace. In Old Testament times this was especially true from the days of King Solomon. Rich people and important people rode on this animal (as did the Judges and their sons). White donkeys were used by persons of high rank. "Speak, ye that ride on white asses, ye that sit in judgment, and walk by the way" (Judges 5:10). These white donkeys are used today in many places in the East by people of high social standing. They are usually larger animals and are supposed to be swifter.”
With regard to Jesus entering Jerusalem on the back of a donkey colt, the article makes this comment:
“Here the use by JESUS of the donkey was to signify that He was Prince of Peace, rather than Captain of an army, when He entered the Holy City [Manners And Customs of Bible Lands]