Jamieson, Fausset and Brown on Joel 2:4

4. appearance ... of horses--(Re 9:7). Not literal, but figurative locusts. The fifth trumpet, or first woe, in the parallel passage (Re 9:1-11), cannot be literal: for in Re 9:11 it is said, "they had a king over them, the angel of the bottomless pit"--in the Hebrew, Abaddon ("destroyer"), but in the Greek, Apollyon--and (Re 9:7) "on their heads were as it were crowns like gold, and their faces were as the faces of men." Compare Joe 2:11, "the day of the Lord ... great and very terrible"; implying their ultimate reference to be connected with Messiah's second coming in judgment. The locust's head is so like that of a horse that the Italians call it cavalette. Compare Job 39:20, "the horse ... as the grasshopper," or locust.

run--The locust bounds, not unlike the horse's gallop, raising and letting down together the two front feet.