A close relative to the dragon, there is a native Korean mythological creature known as an Imugi. By one account, they are hornless creatures resembling dragons who have been cursed and thus were unable to become dragons. By other accounts, an Imugi is a proto-dragon which must survive one thousand years in order to become a fully fledged dragon. In either case they are said to be large, benevolent, python-like creatures that live in water or caves. Their sighting is associated with good luck.
In the 21st-century Korean film titled D-War, two Imoogi, of which one was benevolent and the other evil, were seen competing for possession of a source of power by which one of them could become a dragon. Ultimately, the evil Imoogi is destroyed by his rival moments after the latter had captured the source. Here, the two are shown to be physically different, in that the evil Imoogi is darker-colored, more slender, and distinguished by an inflexible hood similar to that of a cobra, whereas the good Imoogi is paler, stockier, and hoodless. Narration in the film implies that many Imoogi exist at a time, whereof one is designated to become a dragon.
The Alosha series by Christopher Pike features a variation of the Imoogi called a "koul". A koul is a snakelike, gigantic proto-dragon that must pass three tests of courage — the coming to aid of others, the act of swimming, and a literal 'leap' of faith — in order to become a dragon. Upon coming to the aid of others, the koul grows legs; after having swum in water, the koul is able to breathe fire; upon making the leap, the koul immediately sprouts wings. A successful koul resembles a European dragon, whereas a koul who has completed only one or two of its tests resembles a Chinese dragon or Korean dragon of the type described above.
(picture from d-day this is the evil imoogi)