Joel compared this advancing army to warhorses and chariots, the war machines of his day. He heard the familiar sound of chariots in battle, which he likened to the crackling of fire as it rages up a mountainside swiftly consuming everything in its path. The huge army that Joel saw appeared unstoppable. [source][source][source]
It is interesting that locusts look like tiny armored horses, and they behave like them as well (cf. Job 39:19-20; Revelation 9:7). The Italian word for locust means "little horse," and the German word means "hay horse." [1] Thus the correspondence between the army of locusts that had recently swept through the land swiftly and this future invading army is unmistakable. Even their sounds were similar. However, the point of the comparison is probably the horse as a symbol of power and might (cf. Isaiah 31:1-3; Hosea 14:3; Micah 5:10; Haggai 2:22; Zechariah 9:10; Zechariah 12:4; Revelation 9:7). [2][source]