Verse 19: "For the power of the horses is in their mouths and in their tails; their tails are like serpents, with heads, and by means of them they wound."

These are not horses at all. They look more like a snake, that propels itself with his tail, and it wounds by the snake's head that is deadly. This is what John sees in his vision: a nuclear missile that is propelled by its tail and the head ("by means of them they wound") is a nuclear bomb—the warhead.

"the horses" represent the ballistic missiles with their thermonuclear warhead.

"the riders" are the nuclear warheads themselves.

Their "breastplates" represent the results that will come from the explosions of the missiles. Their colors are the colors of the mushroom clouds and represent the radiation, heat, and percussion of the explosions. These are the "three plaques" by which "a third of mankind was killed." Death from any one of these three things is instantaneous.

The "horses" are likened to a serpent in that by its tail a serpent propels itself, and with its head it wounds. The missiles are propelled by their tail and the warheads are the heads which "wound."