I thought this was a charming folk myth and wanted to share it.
Korean folk mythology states that most dragons were originally imugis (Korean: 이무기; RR: Imugi), or lesser dragons, which were said to resemble gigantic serpents. There are a few different versions of Korean folklore that describe both what imugis are and how they aspire to become full-fledged dragons. Koreans thought that an imugi could become a true dragon, or yong or mireu, if it caught a Yeouiju which had fallen from heaven. Another explanation states 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, and their sighting is associated with good luck.
I love the notion that even an imugi can become a dragon if it lives long enough. "Beware of an old man in a profession where men usually die young."