Dedicated to the Lord Jesus Christ Who loved us, gave Himself for us, and washed us from our sins in His own Blood; and to the Church, which is His body.

John 12:14-15 is the fulfillment of Zechariah's prophecy in Zechariah 9:9. Zechariah's prophecy is specific and how exactly it was fulfilled. 
Zechariah prophesied Jesus not only riding an donkey, but also a colt (unbroken), the foal of an donkey. Zechariah also mentioned the people rejoicing greatly and shouting which certainly came to pass on this day.
All four gospels include a triumphal entry, but only Matthew records a donkey with a colt. A simple explanation to a so-called "contradiction" is that Jesus rode the colt while the other donkey went along. No doubt, He rode each animal part of the way.
Not only was it a miracle that Jesus knew about the donkey and its colt, but also where they would be. God also worked some kind of miracle in the owner of these animals so that he would be willing to release them. It is possible that the Lord also revealed to this man that Jesus would need his animals.
Perhaps he was just a devoted follower of Jesus who gladly surrendered them when he knew Jesus was the one wanting them. Either way, it was just as much a miracle that the owner was willing to surrender them as it was that Jesus knew exactly where they would be.
Jesus hadn't been in Jerusalem in quite a while and there is no indication that He had made previous arrangements with anyone there to obtain this dinkey and its colt. This was nothing less than supernatural knowledge imparted to Jesus through the Holy Spirit. As Jesus did, let God's gifts flow through us.