John Gill 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)

All this was done
The disciples were sent to the neighbouring village for the ass and colt, and they brought them, and Christ rode upon them; not because of the distance of the place from Jerusalem, for he was just at it; or because he was weary, or it would be very fatiguing to him to walk thither on foot; for he had been used to travelling, and had gone through most parts of Galilee and Judea; but

that it might be fulfilled which was spoken by the prophet :
the Ethiopic version adds, Isaiah; for the former part of the following citation stands in (Isaiah 62:11) as the latter does in (Zechariah 9:9) . It was usual with the Jews to cite Scripture in this manner, by taking a part from one writer, and another from anther, and joining them together: saying, the following words.