English uses one word for two creatures with different ancestries, different characters and different jobs, and almost every confused statement about dragons comes from the merger.
The European dragon is an adversary. It descends from the Greek drakon, a word meaning a large serpent and connected to a verb for looking or watching, the guardian snake of the Golden Fleece and of the apples of the Hesperides. It absorbs the chaos-serpents of the ancient Near East: Tiamat in the Enuma Elish, Leviathan in Job and Psalms, Lotan in the Ugaritic texts. Revelation 12 makes the identification explicit, calling the great dragon that ancient serpent, the devil and Satan. From there the dragon in Christian Europe is unambiguously an enemy, and the standard story is a killing: Beowulf's barrow-dragon, guarding treasure and roused by theft; Sigurd and Fafnir; Saint George, whose dragon fight is not part of the earliest accounts of the saint and reaches the West largely through the Golden Legend of Jacobus de Voragine in the thirteenth century.
The East Asian long is a benefactor. It governs rain, rivers and the sea, it is invoked in drought, and it is the emblem of imperial authority in China from the Han period, with the five-clawed dragon reserved to the emperor by law in later dynasties. Classical descriptions compose it from the parts of nine other animals, camel's head, deer's antlers, carp's scales, eagle's talons, and it is generally wingless, flying without needing them. The Japanese ryu, Korean yong and Vietnamese rong belong to the same family.
Reading them as one creature makes the East Asian dragon look like a benign variant of a monster. It is not a variant of anything.