John Gill 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 brought the ass and the colt
To Jesus, as Mark and Luke add, and who only make mention of the colt: both were undoubtedly brought; the colt being unloosed and taken away, the ass, its dam, followed after:

and put on them their clothes ;
their loose upper garments, to be instead of saddles and trappings, and that Christ might sit thereon with ease and decency: the other evangelists say, that they cast their garments on the colt; and the Syriac version here reads, "they put their garments on the colt, and Jesus rode upon it": but as both were brought, it is clear from hence, that their clothes were put upon both; not knowing which Christ would choose to ride on. And it should seem, that it was not unusual to put garments on asses to ride on; for the Targumist on (Judges 5:10) represents the princes of Israel as riding upon asses, strewed or saddled with all kind (Nyrwyu) , of "painted garments". The Persic version, without the least colour of authority from the original text, renders it, "and Jesus put his own garment on the colt, and sat thereon"; which is ridiculous, as well as contrary to truth:

and they sat him thereon ,
or "on them": meaning either on the ass and colt, that is, on one of them, or both successively, or on the clothes they put upon them.