Revelation 9:17

Now John saw what these horsemen looked like. Horses were swift implements of war in ancient times. Red, blue, and yellow breastplates covered both horses and riders. This was apparently their only armor, and it is defensive armor. Hyacinth is a flower that is most commonly blue, and brimstone is sulfur that is yellow. Lion-like horse heads could be very different from those of ordinary horses or just heads of horses that appear exceptionally bold and majestic. Lions are terrifying (cf. Revelation 10:3), fierce (cf. Revelation 9:8), and destructive (cf. Revelation 13:2). Natural horses do not breath fire, smoke, and brimstone. These may be figures describing their prophecies of judgment (cf. Revelation 11:5-6). This verse suggests that this army is probably something other than a human army of cavalry, probably an angelic army (cf. Revelation 9:9). However some interpreters, both premillennial and amillennial, have believed that this will be a human army. [Note: E.g., Walvoord, The Revelation . . ., p. 167; and Hendriksen, p. 148.]