One of the amazing things about Palm Sunday is the contrast between the humility of riding a donkey, and the majesty of who Jesus really is.
Zechariah 9:9 is a key verse which helps us understand: "Rejoice greatly, O Daughter of Zion! Shout, Daughter of Jerusalem! See, your king comes to you, righteous and having salvation, gentle and riding on a donkey..." (this is quoted in the Palm Sunday account in Matthew 21:1-11)
WOW! Time to sing "Hosanna!" How come the high priests were so grumpy?!