Matthew 21:2-7 and Mark 11:2-7; Luke 19:30
Donkey and colt (Matthew 21:2-7) - "Go into the village opposite you, and immediately you will find a donkey tied there and a colt with her; untie them, and bring them to Me. 3“And if anyone says something to you, you shall say, ‘The Lord has need of them,’ and immediately he will send them.” 4Now this took place that what was spoken through the prophet might be fulfilled, saying, 5“Say to the daughter of Zion, ‘Behold your King is coming to you, gentle, and mounted on a donkey, even on a colt, the foal of a beast of burden.’” 6And the disciples went and did just as Jesus had directed them, 7and brought the donkey and the colt, and laid on them their garments, on which He sat."
A colt (Mark 11:2-7) - "Go into the village opposite you, and immediately as you enter it, you will find a colt tied there, on which no one yet has ever sat; untie it and bring it here. 3"And if anyone says to you, ‘Why are you doing this?’ you say, ‘The Lord has need of it’; and immediately he will send it back here." 4And they went away and found a colt tied at the door outside in the street; and they *untied it. 5And some of the bystanders were saying to them, "What are you doing, untying the colt?" 6And they spoke to them just as Jesus had told them, and they gave them permission. 7And they *brought the colt to Jesus and put their garments on it; and He sat upon it."
A colt (Luke 19:30) - "Go into the village opposite you, in which as you enter you will find a colt tied, on which no one yet has ever sat; untie it, and bring it here."
There is no contradiction. Matthew 21:2-7 tells us that there was both a donkey and a colt. Mark and Luke focus on the colt only and mention that no one had ever sat upon it. Mark and Luke are focusing on this detail while Matthew focuses on the prophetic fulfillment (Matthew 21:4-5). Logically, if there are two animals, then there is also, at least, one animal. To say there was one does not mean there weren't two. This is not a verbal game. It is an issue of logic. Remember, the writers of the gospels wrote for a purpose. It was not to recount a chronologically precise account in minute details. It was to convey the validity of Christ. The fact that Mark and Luke mention one colt does not mean there is a contradiction anymore than saying that Frank and Joe came to my house last night but today I tell a friend about what Joe said last night and don't mention Frank.
Zechariah 9:9 is the scripture that Matthew refers to. It says, "Rejoice greatly, O daughter of Zion! Shout in triumph, O daughter of Jerusalem! Behold, your king is coming to you; he is just and endowed with salvation, humble, and mounted on a donkey, even on a colt, the foal of a donkey." We can see that Matthew is simply including both animals as was prophesied in Zechariah.
Why would both be needed if Jesus only rode one into Jerusalem? The simple answer is that the colt was young and still attached to the mother, and vice versa. They would travel together as a mother and offspring naturally would among many animal species.
Finally, the disciples did not steal the colt. Matthew states that the person who owned the animals would send them after the disciples stated that "The Lord has need of them." This means it was a voluntary action of the owners of the animals. Certainly, Jesus would not advocate stealing.