John tells us in Revelation 9:17 that the 200,000,000 horsemen had breastplates of fire, jacinth and brimstone. It was Paul who encouraged believers to put on the breastplate of righteousness (Ephesians 6:14) and the breastplate of faith and love (1Thessalonians 5:8), but the breastplates of these horsemen were of fire, jacinth and brimstone (Revelation 9:17). What does all of this mean?
The Greek word translated jacinth (G5191) is “a red color bordering on black,” according to Thayer’s Greek Lexicon, probably meaning smoke, as is seen later in the verse and in Revelation 9:18. This is the kind of thing that was seen in the Lord’s overthrow of Sodom and Gomorrah (Genesis 19:24), which stands as a symbol of what his judgment would be like against any nation that would sin as they did (Deuteronomy 29:23; Isaiah 1:9-10; 3:9; 13:19; Jeremiah 23:14; 49:18; 50:40). Indeed, when the Jews rebelled against God, they were likened to Sodom (Deuteronomy 32:32; Ezekiel 16:46, 48-49, 53, 55-56; Revelation 11:8). The believers’ armor is made up of righteousness, faith and love, while the breastplates of these horsemen were made up of judgment, itself (2Peter 3:7, 11-12).
The heads of the horses were like the heads of lions, and out of their mouths came fire, smoke and brimstone (Revelation 9:17).The significance of the fire, smoke and brimstone that issued from the mouths of the horses (Revelation 9:18) is that by these one-third of men were slain. It is not one-third of mankind, as in some translations, but one-third of men, which, when seen in the context of the Lord coming in judgment against the land of the Jews, it is one-third of the Jews who were slain.
According to John, the power of the horses was in their mouths and their tails, because their tails were like serpents with heads on the ends, which have power to hurt men (Revelation 9:19). The Greek word used for hurt or harm (G91) has to do with injustice or wickedness. Such things are the acts of men. While the Lord might incorporate such a thing in his judgment, he would never do or command such a thing to judge men. Nevertheless, instead of intervening against the injustice committed, he would allow it to occur and incorporate it into his judgment against the Jewish state that he had set his face against.
John tells us that the power of the army of God was in their mouths, out of which issued fire, smoke and brimstone (judgment). This was realized in the persons of the Roman army coming against the Jewish state in 70 AD. However, there is another force to be reckoned with, and that is the serpentine tails that had heads. This force had to do with wickedness, so while not originally planned to be a part of the Lord’s judgment, it was incorporated into that judgment. The tail points to an inferior authority (cf. Isaiah 9:15), but how can we identify that authority?
“Tail” = inferiority, as opposed to “head” = superiority, leadership. “Yahweh will make you the head, and not the tail; and you will be above only, and you will not be beneath; if you will hearken unto the commandments of Yahweh” (Deuteronomy 28:13; compare also Deuteronomy 28:44).
Viewed in this fashion leads me to believe that on the one hand the Jews were judged by God through a superior authority, i.e. the Romans (cir. 70 AD). However, in addition to this an inferior authority added its venom against Jewish state. Therefore, in a manner of speaking the unrepentant Jewish state was doubly judged (cf. Isaiah 61:7; Jeremiah 17:18). In this context we are able to understand that, in the latter days of the nation of Israel, they would become totally corrupt (Deuteronomy 31:29) and completely abandon the Lord, as is seen in their rejection of Jesus, their Messiah and Lord. Therefore, their wickedness had brought upon themselves the curses Moses rehearsed before them:
Deuteronomy 28:15 But it shall come about, if you do not obey the LORD your God, to observe to do all His commandments and His statutes with which I charge you today, that all these curses will come upon you and overtake you: (41) You shall have sons and daughters but they will not be yours, for they will go into captivity. (42) The cricket shall possess all your trees and the produce of your ground. (43) The alien who is among you shall rise above you higher and higher, but you will go down lower and lower. (44) He shall lend to you, but you will not lend to him; he shall be the head, and you will be the tail. (45) So all these curses shall come on you and pursue you and overtake you until you are destroyed… (emphasis mine)
The tails of the Lord’s army were the Jewish leadership themselves. They were brought low and became the tail. Their tails resembled serpents, and in the context of Revelation 9:19, it was the leaders of the Jewish nation who led the Jews astray (cf. Isaiah 9:15). Both John the Baptist and Jesus referred to the Pharisees, scribes and Sadducees as vipers, which are serpents (Matthew 3:7; 12:34; 23:33), and Jesus, himself, said the Pharisees refused to enter the Kingdom of God and prevented those who were trying to enter (Matthew 23:13), and in preventing the general public from entering the Kingdom of God, the Jewish leadership led the nation away from salvation and into the deathtrap of judgment.
 See International Standard Bible Encyclopedia (ISBE) under “Tail” (figurative).