Theophylact of Ohrid

Luke and Mark mention only one beast of burden, but Matthew mentions both ass and foal; yet they do not contradict. The mother followed the foal which was led. "He sat thereon," not on the two beasts, but on the clothes. Or, first He sat on the ass and later on the foal, since He first took His rest in the synagogue of the Jews and later among the people of the Gentiles.

Commentary on Matthew ↗

Jerome

However, the disciples went and did as Jesus had ordered them. They brought the donkey and the colt, and put their cloaks on them, and Jesus sat on them. This colt and donkey, on which the apostles spread their cloaks so that Jesus might sit more comfortably, were naked before the coming of the Savior, and they shivered without cover, with many claiming dominion over them. But after they received the apostolic garments, they became more beautiful and had the Lord as their rider. …

Commentary on Matthew ↗

Hilary of Poitiers

All of these circumstances surrounding his appearance point to the shape of the future. By means of parabolic signs and by the conditions of present things, the form of the future is here suggested. The Lord is taking possession of the nations! His brightness is approaching! He is occupying the minds of the nations—just as the rider of a beast. He is proclaimed by the entire army of his retinue of patriarchs, prophets and apostles. The patriarchs are spreading their garments, which signify their glory, under the Lord. …

Commentary on Matthew 21.2 ↗

Thomas Aquinas

"And the disciples going, did as Jesus commanded them." After the command has been presented, here the execution of the command is presented. And first in general: "the disciples going." Behold, the command of obedience is given. Exod 29:35: "All the things the Lord hath commanded, we will do."

Commentary on Matthew ↗