13 And the sixth angel sounded, and I heard a voice from the four horns of the golden altar which is before God, 14 one saying to the sixth angel who had the trumpet, “Release the four angels who are bound at the great river Euphrates.” 15 And the four angels, who had been prepared for the hour and day and month and year, were released, so that they might kill a third of mankind. 16 And the number of the armies of the horsemen was two hundred million; I heard the number of them. 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 bbrimstone; and the heads of the horses are like the heads of lions; and out of their mouths proceed fire and smoke and bbrimstone. 18 A third of mankind was killed by these three plagues, by the fire and the smoke and the brimstone, which proceeded out of their mouths. 19 For the power of the horses is in their mouths and in their tails; for their tails are like serpents and have heads; and with them they do harm. 20 And the rest of mankind, who were not killed by these plagues, did not repent of the works of their hands, so as not to worship demons, and the idols of gold and of silver and of brass and of stone and of wood, which can neither see nor hear nor walk; 21 and they did not repent of their murders nor of their sorceries nor of their immorality nor of their thefts.
Note: Biblically speaking "horses" mean something to ride on. With the onset of robotics and their great sophistication this is most likely how this prophecy can become reality.