9:17 And this is how I saw in the vision the horses and those who sat on them: the riders had breastplates the color of fire and of hyacinth and of brimstone; and the heads of the horses are like the heads of lions; and out of their mouths proceed fire and smoke and brimstone.

And this is how I saw in the vision the HORSES and THOSE who sat on them:

How do you describe something that is spiritual (evil/demonic, in this case)? Most of us live so closely to the physical world that we seldom experience the spiritual world. And when we do experience the spirtual event, it has been my experience that the event is very real to you but hard to put into words. This sounds like the case for John in this verse. Since, most of us have seen a horse with a rider, that is how John starts.

1. the RIDERS had breastplates the color of fire and of hyacinth and of brimstone; and

This part John knew for sure. The RIDERS had colorful protective gear of reds, yellows, and blues. What is never described in Revelation is the smells of these visions other than the incense (prayers, Revelation 8:4). And even this is left to our imagination.

Fire and brimestone > God's Judgment
Sodom and Gomorrah Genesis 19:24
Idol-worshipping country Deuteronomy 29:23
Bareness or lack of plenty Job 18:5-15-21
"Portion of wicked's cup" Psalm 11:4-6-7
Breath of the Lord Isaiah 30:27-33
Judgment Isaiah 34:1-9-15
Prophesy against Gog Ezekiel 38:17-22-23

Hyacinth may be the color of the "smoke" of the horses kicking up "dust" as they stomp through mankind and kill them. See that "SMOKE" is inserted between fire and brimestone in the next verse.

2. the HEADS of the HORSES are like the heads of lions; and

This now is symbolic since the word 'like' is used. Not only are these horsemen protected by breastplates, but they are fearless and powerful like lions. Some scholars who are looking at a physical interpretation indicate this may be a special kind of military tank. However, this probably is the devil prancing around (1 Peter 5:8) trying to imitate the real LION (Revelation 5:5).

3. out of their MOUTHS proceed FIRE and SMOKE and BRIMSTONE.

The FIRE and SMOKE and BRIMSTONE is proceeding out of the MOUTHS of the riders. These are three awful destructors or testers.

Fire has always been used to test a man's work. Otherwise, why would we have "fire departments" to try to save our earthly belongings.

Smoke is a spirit/demon activity. It is used both to describe evil spirit activity and the strength activity of God.

Brimstone lays waste. It is like lava; a third of mankind is "melted down".