Their appearance is as the appearance of horses, and as horsemen, so do they run.
The locusts' heads resemble horses closely, a fact observers have long noted. Their springing motion roars like war chariots on rough roads; their devouring crackles like fire on stubble. The nations see this army of God approach and their faces go pale with terror.
AI summary
Commenting on Joel 2:4-6
appearance . . . of horses-- (Rev 9:7). Not literal, but figurative locusts. The fifth trumpet, or first woe, in the parallel passage (Rev 9:1-11), cannot be literal: for in Rev 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 (Rev 9:7) "on their heads were as it were...
The appearance of them is as the appearance of horses,.... in their running, as Jarchi and Kimchi interpret it; they came with, as much swiftness and eagerness, with as much fierceness and courage, as horses rush into the battle. Bochart (h) has shown, from various writers, that the head of a locust is in shape like that of a horse; and Theodoret on the text...