Hilary of Poitiers — as excerpted in the Catena Aurea on Matthew 21:1-9

“Or by the ass and the colt is shewn 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.”