In a battle involving horses, the biggest danger in relation to the animal is its feet. But in this case the weapon these horses use is their mouths. In Scripture what comes out of a mouth are words, teaching, instruction (see Revelation 1:16). These horses fight by uttering life‑choking instruction. Note that the reference to their mouths is repeated (thus, underlined) in Revelation 9:19.
17 And this is how I saw the horses in my vision and those who rode them: they wore breastplates the color of fire and of sapphire and of sulfur, and the heads of the horses were like lions’ heads, and fire and smoke and sulfur came out of their mouths.