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 Et ainsi dans la vision je vis les chevaux et ceux qui les montaient, ayant des cuirasses couleur de feu, d'hyacinthe, et de soufre. Les têtes des chevaux étaient comme des têtes de lions; et de leur bouche il sortait du feu, de la fumée, et du soufre.