Hilary of Poitiers

Or by the ass and the colt is shown the twofold calling from among the Gentiles. For the Samaritans did serve after a certain fashion of obedience, and they are signified by the ass; but the other Gentiles wild and unbroken are signified by the colt. Therefore two are sent to loose them that are bound by the chains of error; Samaria believed through Philip, and Cornelius as the first-fruits of the Gentiles was brought by Peter to Christ.

Catena Aurea by Aquinas ↗

Thomas Aquinas

And he does three things. First, he commands them on a salutary mission; secondly, he gives a command about salvation; thirdly, about contradicting. He says therefore "go into the village that is over against you." Literally, there was a certain village which was opposite, to signify the world into which the Lord sent them. Mark 16:15: "Going into the whole world, preach the Gospel to every creature." And it will be against them. …

Commentary on Matthew ↗