John Lightfoot on Mark 5:9

[My name is Legion.] I. This name speaks a numerous company, the devil himself being the interpreter; "Legion (saith he) is my name, for we are many."

And among the Jews, when a man would express a great number of any thing, it was not unusual to name a legion: "R. Eliezer Ben Simeon saith, It is easier for a man to nourish a legion of olives in Galilee, than to bring up one child in the land of Israel."

II. Among the Talmudists, a legion bespeaks an unclean company; at least, they reckoned all the legions for unclean: "The Rabbins deliver: a legion that passeth from place to place, if it enter into any house, the house is thereby become unclean. For there is no legion which hath not some carcaphalia. And wonder not at this, when the carcaphalion of R. Ismael was fastened to the heads of kings." "'Carcaphal' (saith the Gloss) is the skin of a head pulled off from a dead person, which they make use of in enchantments."

III. What the Romans thought of their legions, take from the words of Caesar to the Spaniards: "Did ye not consider, if I were overthrown, that the people of Rome have ten legions, which could not only resist you, but pull down even heaven itself?" What then is the power of "more than twelve legions of angels"!