We are meant to understand that the horses represent the invasion of heathendom into the (non‑heathen) world. By their mouths they present their pagan worldview to humanity. At the same time there is something hellish about these horses, as is evident from the hellish atmosphere coming from their mouths (see also Revelation 14:10–11; Revelation 19:20).
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.