The dragons

Let me tell about dragons first, (because I like them very much!) There is difference with the West and the East especially China. (There is some evil dragons in the East, but I think it is influenced by the Western one.) In the West, it is told as evil. They has treasure, attack people, and sometimes carry off women. They are hunted by hero at last. However in China, they are worshiped. They are posted as God, and their appearance mean good symbol. Though these difference, it commonly used as symbol to show oneself power in both of them. They are different in their own figure. In the West, they similar to lizard. They have large body, four thick legs to walk and wings. But in China, they similar to snake. Their body is thin and long. They have four legs too, but it isn't thick as Westernone. They fly without wings as swim.
Are both is surely dragon? With these difference? I wonder that why person ― I don't know it's Western of Eastern ― defined both are same "dragon." Why he or she thought dragon and "Ryuu" are same thing? If I could know it, it's wonderful I think.