The portrayal of the "dragon" originates from the "serpent" images in the OT, but is used in Revelation as the devouring predator enemy of Christ.
Excerpt from my post "The Signs of Revelation - Part V: The Animal Symbols of The Battle":
3. Dragon – Rev. 12:3, 4, 7, 9, 13, 16, 17; 13:2, 4, 11; 16:13; 20:2 – the devil, Satan, the serpent of old from Genesis. chap. 3.
The word “dragon” is drakon in the Greek, and means a huge serpent. It only appears in Revelation. In the OT, it is the serpent in Ex. 7:9-10; the representation of Nebuchadnezzar as a dragon in Jer. 51:34; the representation of the idolatrous tribes of Israel in Deu. 32:33; the representation of Babylon in Isa. 27:1; 51:9; and the representation of Pharaoh, king of Egypt in Ez. 29:3.
The symbolic use of “dragons” and “serpents” meant the pagan, heathen nations who worshipped and sacrificed to idols, the unclean things they made with their own hands. Those pagan, idolatrous nations were always the enemies of the Most High.
Rev. Chap. 12 opens with a flash back to the birth of Christ and His church in the first part of the first century A.D, and is the fulfillment of Is. 26:17-21. Remember that Revelation is not a chronological account, but rather a series of themes, spot-lighting the events, recounting the prophesy from different aspects much as an historian would recount a war from different battle fronts.
Chapters 4 – 11 were presented and originated from the throne of God with the ascension of Christ, His judgments, and His leadership of the battle to be fought against the enemies of God. With Chapter 12 the prophesy turns to the battle that Satan and his agents / messengers brought against Christ and His church, His bride, the new Jerusalem.
The dragon in Revelation was a representation of the world power that was the enemy of Christ and His church. It appeared in “heaven”, the sphere of the political world dominion and power of Rome, the empire which ruled over Palestine, the “earth.” (See Part II for the meanings of “earth,” and “heaven” as used in prophesy.)
Satan was personified in the Caesars of Rome, and in the Sanhedrin of Judah who persecuted the saints and Christ’s church. This was the battle between heathendom, pagan idolatrous nations which was prophesied in Ez. chap. 38, with the enemies of God (Gog, the prince of the land of Magog) who were trying to obliterate Christianity. See here for a good discussion of Gog and Magog.
The seven heads and ten horns of the dragon correspond to the seven heads and ten horns of the sea beast in Rev. 13.1 identifying them as the same world power.
” And God [is] my king of old, Working salvation in the midst of the earth. 13 Thou hast broken by Thy strength a sea-[monster], Thou hast shivered Heads of dragons by the waters, 14 Thou hast broken the heads of leviathan, Thou makest him food, For the people of the dry places.” (YLT)
Satan gave power to the beast to make war against the saints. (Rev. 13:4) He knew his time was short. (Rev. 12:12) His work and power accumulated through the pagan and idolatrous kingdoms of Babylon, Medo-Persian, and Greek empires and were culminating in that of the Roman empire. If Satan could not stop Christianity then and there, he would never be able to do so. Of course, he was never going to win. (Psa. 98:1; Is. 25:7-9; Dan. 12:1-3)
The dragon’s tail swept away a third of the “stars of heaven” in v. 4 and cast them down to “earth.” Remember from Part II that the “stars of heaven” were the sons of Israel, or the Jews. This is a reference back to Rev. 8:7-12 with the third parts harmed under the four angels. It refers to the Jewish authorities, and apostate people of Judah and Jerusalem.
The symbolic vision in v. 4 of the dragon standing before the woman ready to devour her child as soon as it was born is the image of both the Virgin Mary and the birth of Christ (Is. 66:7), who then fled to avoid the slaughter of the innocents under Herod (Matt.2:13); and also stands for the mother church born on the day of Pentecost (Acts 2:38-41) which later fled into the wilderness mountains to survive and be sustained during the persecution and tribulation of the Roman/Judean war. (See Matt. 24:16-20; Mark 13:14-18; Luke 21:20-22)
The Roman Caesars were claiming to be gods, and sons of gods. Their claims were those of impostors, who forced the people to worship them through acts of terror. Those that succumbed to the “mark of the beast” were those that worshiped the Caesars of Rome, and other pagan “gods.”
Satan’s desire was always to deceive the nations / people so that they would turn away from the one, true God. This was the war that was prophesied in Gen. 3:15 to be fought between the Seed of the woman (Christ) and the seed of the serpent. This battle waged by Satan was present throughout all of the OT. He desperately wanted to prevent Christ’s birth, and then to destroy Him.
The battle between the dragon and his “angels” – messengers / agents / followers – and Michael and His “angels” was fought in the “heaven” of the political, world-rule of the Roman empire.
Michael was the chief of the angels, and Daniel’s “prince” in Dan. 10:13, 21. Michael was the “great prince” who was prophesied to stand up for Daniel’s people in Dan. 12:1 during the time of tribulation, and the Archangel of Jude 9 who contended with the devil for Moses’ body. The word “Archangel” is capitalized, therefore a specific angel, and means the ruler of the angels. The only other time this word is used is in 1 Thess. 4:16 when Christ descends from heaven with the voice of the archangel.
Michael of the OT was the prefigured Christ, and the warrior, the Prince of princes (Dan. 8:25) who fought for His people, and is used again in Revelation as the symbol of Christ, the warrior on the white horse. Christ and the apostles and the newly converted saints were the ones battling with Satan and his angels. The battle was fought on the “earth” of Jerusalem, Judea, and Palestine. (Rev. 12:13-17)
There was no battle in the heaven where God sits on His throne. There is no evil, nor apostasy in heaven above! Therefore, this battle in Rev. 12:7-8 was symbolic of the spiritual struggle of principalities over the newly born church of Christ.
It took place during the time when the “woman,” the church fled into the wilderness in v. 6 where she hid for the 3-1/2 years (time, times, and a half / or 1260 days) which Christ told his disciples to do in Matt. 24:15-20. This was the 42 months of the Judean / Roman war when the gentile nations trampled the “outer court” of the temple under their feet (Rev. 11:2), and which ended with the destruction of that old Jewish temple in Sept. A.D. 70.
Thus, the dragon, Satan, and his emissaries / angels were defeated, and fell from their positions of power down to “earth.” (Rev. 12:8-9) Revelation chap. 12 is a preview of the battle that is portrayed in more detail throughout chapters 13 -19." Posted here
The language of prophesy needs to be studied in order to understand the metaphors. The dragon / serpent imagery was always used for the Adversary even from the garden of Eden in Gen. chap. 3.
All bold emphasis is mine.