A.T. Robertson Commentary Revelation 9:17

"And thus I saw the horses in the vision, and them that sat on them, having breastplates [as] of fire and of hyacinth and of brimstone: and the heads of lions; and out of their mouths proceedeth fire and smoke and brimstone." — Revelation 9:17 (ASV)

And thus I saw in the vision (κα ουτως ειδον εν τη ορασε). Nowhere else does John allude to his own vision, though often in Dan. (Daniel 7:2; 8:2,15; Daniel 9:21).

Having (εχοντας). Accusative masculine plural of εχω, probably referring to the riders (τους καθημενους επ' αυτων) rather than to the horses (τους ιππους).

Breastplates as of fire and of hyacinth and of brimstone (θωρακας πυρινους κα υακινθινους κα θειωδεις). There is no ως (as) in the Greek, but that is the idea of these three adjectives which are only metaphors. Πυρινος is an old adjective (from πυρ, fire), here only in N.T. Hυακινθος is also an old word (from υακινθος, hyacinth, then of a sapphire stone Re 21:20), of a red color bordering on black, here only in the N.T. Θειωδης is a late word (from θειον, brimstone), sulphurous, here only in N.T.

As the heads of lions (ως κεφαλα λεοντων). This of the horses, war-horses as always in the Bible except in Isa 28:28. These horses likewise have "fire and smoke and brimstone" (θειον, brimstone, is old word, in N.T. only in Rev. and Lu 17:29) proceeding (εκπορευετα, singular because it comes first and the subjects afterwards) out of their mouths. Both rider and horse are terrible.