(de Con. Ev. ii. 66) Matthew says, an ass and a colt, the rest however do not mention the ass. Where then both may be the case, there is no disagreement, though one Evangelist mentions one thing, and a second mentions another; how much less should a question be raised, when one mentions one, and another mentions that same one and another. It goes on: And certain of them that stood there said unto them, What do ye, loosing the colt? And they said unto them even as Jesus had commanded, and they let them take it, that is, the colt.
Source: Catena Aurea by Aquinas