A.T. Robertson Commentary Matthew 21:7

"and brought the ass, and the colt, and put on them their garments; and he sat thereon." — Matthew 21:7 (ASV)

And he sat thereon (κα επεκαθισεν επανω αυτων), Mark and Luke (Luke 19:35) show that Jesus rode the colt. Matthew does not contradict that, referring to the garments (τα ιματια) put on the colt by "them" (αυτων). not to the two asses. The construction is somewhat loose, but intelligible. The garments thrown on the animals were the outer garments (ιματια), Jesus "took his seat" (επεκαθισεν, ingressive aorist active) upon the garments.