Well I don't think there is a matter of linguistics, given that the verse in Zechariah (yes, there are prophets other than Isaiah that talked about the Messiah! Go and read them!) reads (in the NRSV):
'...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.'
(slightly different in Matthew 21:5, as Matthew is probably quoting the Septuagint). There's no 'and' in sight. And when Matthew is clearly expanding on Mark, who has Jesus mounted simply on a colt, why else the addition of a donkey as well except to really beat the reader over the head with Zechariah? (okay, now that is a fun mental image)
As for hermeneutics in general, Nathan is spot on. I am not 'casting aspersions' on Matthew, and I rather resent the implications of that statement, brother. I believe that textual criticism is a valuable tool in discerning what the Bible is telling us about Jesus, and that God wouldn't have given me a half-worthy brain and a hefty shove in the direction of academic Theology if he intended me to sit there with my fingers in my ears. Christianity is the truth, and as such has absolutely nothing to fear from intellectual enquiry; if intellectual enquiry bids us lay down some of the false assumptions that we have accumulated around our Christianity (as I have had to do) then praise God for that.