A.T. Robertson Commentary Matthew 21:4

"Now this is come to pass, that it might be fulfilled which was spoken through the prophet, saying," — Matthew 21:4 (ASV)

By the prophet (δια του προφητου). The first line is from Isa 62:11, the rest from Zec 9:9. John (John 12:14f.) makes it clear that Jesus did not quote the passage himself. In Matthew it is not so plain, but probably it is his own comment about the incident. It is not Christ's intention to fulfil the prophecy, simply that his conduct did fulfil it.