A.T. Robertson Commentary Revelation 12:3

"And there was seen another sign in heaven: and behold, a great red dragon, having seven heads and ten horns, and upon his heads seven diadems." — Revelation 12:3 (ASV)

Another sign (αλλο σημειον). "A second tableau following close upon the first and inseparable from it" (Swete).

And behold (κα ιδου). As often (4:1; 6:2,5,8, etc.).

A great red dragon (δρακων μεγας πυρρος). Homer uses this old word (probably from δερκομα, to see clearly) for a great monster with three heads coiled like a serpent that ate poisonous herbs. The word occurs also in Hesiod, Pindar, Eschylus. The Babylonians feared a seven-headed hydra and Typhon was the Egyptian dragon who persecuted Osiris. One wonders if these and the Chinese dragons are not race memories of conflicts with the diplodocus and like monsters before their disappearance. Charles notes in the O.T. this monster as the chief enemy of God under such title as Rahab (Isaiah 51:9f.; Job 26:12f.), Behemoth (Job 40:15–24), Leviathan (Isaiah 27:1), the Serpent (Amos 9:2ff.). In Ps 74:13 we read of "the heads of the dragons." On πυρρος (red) see 6:4. Here (12:9) and in 20:2 the great dragon is identified with Satan. See Da 7 for many of the items here, like the ten horns (Daniel 7:7) and hurling the stars (Daniel 8:10). The word occurs in the Apocalypse alone in the N.T.

Seven diadems (επτα διαδηματα). Old word from διαδεω (to bind around), the blue band marked with white with which Persian kings used to bind on the tiara, so a royal crown in contrast with στεφανος (chaplet or wreath like the Latin corona as in 2:10), in N.T. only here, 13:1; 19:12. If Christ as Conqueror has "many diadems," it is not strange that Satan should wear seven .