The devils entered not into the swine of their own will, but their asking for this concession was that is might be shewn that they cannot hurt men without Divine permission. They did not ask to be sent into men, because they saw that He, by whose power they were tortured, bore a human form. Nor did they desire to be sent into the flocks, for they are clean animals offered up in the temple of God. But they desired to be sent into the swine, because no animal is more unclean than a hog, and devils always delight in filthiness.
It goes on: “And forthwith Jesus gave them leave.”
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: see Aurea Catena in Matt., p.327