Matthew 21:5-7: "Tell the daughter of Zion, Look, your king is coming to you, humble, and mounted on a donkey, and on a colt, the foal of a donkey." The disciples went and did as Jesus had directed them; the brought the donkey and the colt, and put their cloaks on them, and he sat on them.
In one of the clearest examples we have we can see how the author of Matthew (who, for simplicity's sake, we'll call "Matthew) goes to amusing lengths to make sure his audience knows Jesus fulfilled all the relavent (in his mind at least) prophecies. The prophecy being referenced here is from Zechariah 9:9:
Rejoice greatly, O daughter Zion! Shout aloud, O daughter Jerusalem! Lo, your king comes to you; triumphant and victorious is he, humble and riding on a donkey, on a colt, the foal of a donkey. (NRSV)
Unlike the parallel texts in Mark 11:1-10 and Luke 19:28-38, Matthew apparently didn't understand the Hebrew parallelism in the original text. Only a single animal was meant. A clear example of this can be seen in Genesis 49:11.
Moreover, Matthew apparently didn't realize the lunacy of saying that Jesus rode both a donkey and a colt simultaneously; he was too concerned with making sure his version of the Jesus story had Jesus fulfilling every single prophecy exactly, because, after all, for Matthew, more so than for any other gospel writer, Jesus is the Jewish messiah, meaning that he had to fulfill all of the Jewish prophecies, even the ones Matthew made up.
What this passage lacks in potential accuracy it makes up for in humor. Though it would be pretty cool for Jesus to be circus Jesus. Maybe the scene looked something like this: