For anyone thinking that these are dinosaurs, there is one problem with that reasoning; It doesn't explain the specificity of the mythology that is associated with the dragons. They are always extremely intelligent, they can speak, they are always associated with gold, they have supernatural powers, and come from worlds or realms beyond are own. They are in mythology associated with very specific animals, usually one with scales, goats (for their horns and beard) and eagle or owl (ability to fly), and almost always a lion. They are the prototype demons of our religions, sometimes from very sophisticated societies. (i.e. lilith from some Adam and Eve stories, Mesopotamia).
Eastern religions see them as more neutral but who deliver hard punishments when breaking the law, Western religions almost always see them as powerful adversaries, or as demons. Often times they are said to have created human kind.