The origin of dragons is ancient man's explanation for monstrous creatures preserved in rock.
Today we call those preserved bones "fossils" but ancient man was... standing upon the shoulders of shorter giants than scientists of today stand upon.
Related: clearly fish creatures preserved in the same rocks... thousands of feet above sea level. Literal experience, not just stories beginning when a child from mom and dad, with sudden, unexplained, massive ice dam breaking floods. Not hard to see how a "biblical flood" mythos developed around the world. In the northern hemisphere anyway. And I've not heard of any in the southern hemisphere, even though that seems like it ought to be included in the phrase one hears of "flood myths are everywhere in the world so there has to be something to them..." No glacial sheets in any of the southern hemisphere except for some reaching Argentina, and most of those before the extent of the Americas was reached by man at least 30,000 years ago.
Various monstrosities, such as the Greek Titans and Cyclopes surely harken from similar, then modern, reconstructions.
People come up with explanations, even when they have no connection whatsoever with what we today know (and "know"... nice thing about science is that as new evidence and explanations better our knowledge, it slowly changes to reflect them... where myths do not much do so).
Interesting though, and clearly diagnostic of cultures in may ways, the main and the minor differences that have taken hold and developed. The "Drakōntas (δράκοντος" brought up by
Walter for instance. What RPG scenario, to pick a living use of dragon mythos, has dragons guarding something of value versus hording something of value normally stolen from many with those many usually becoming "the few" in the process? Not at all duty-bound creatures and definitely not the helpful sort as mentioned in the other question that is mentioned by
cmw in the comments.
Personally, I find it most fascinating that dragons, which must hark back a LONG way, are usually the helpful creature type in Asian mythos, but to all, not just rulers, while we often regard Asian cultures as repressing the individuality of individuals in furtherance of maintaining orderly society so that their very large, historically, populations do not devolve into murderous mobs from the constant angering of each other due to casual self-centered behavior, where that recent (last some thousands of years) society might be thought to have generated dragons who felt a duty to rulers and to have helped the rulers, not people in general, in furtherance of said repression and thereby helping those populations survive themselves. To me, that is diagnostic of such massive populations being a fairly recent thing ("relatively recent" as a fair number of millennia would be covered), and that those much larger than elsewhere in the world supportable populations might not go back as far as most casually assume. That would make the cultural shift to repressing individuality a fairly recent thing as well, as important as the concept of trade expanding populations due not just to now getting a wider range of materials, and later probably, products, but mainly due to using and expanding populations on the back of the gains of comparative advantage. An innovation, over a time period not at all like the modern grind of innovation, but an innovation nonetheless as opposed to an innate characteristic of the people involved, further emphasizing our underlying similarity, and more.
Well, I, um, steer close to digressing...
Dragons are the explanation of dinosaurs and similar creatures preserved as fossils and come upon in many ways by ancient peoples who attempted to explain what clearly absolutely existed at some time. That the explanation was wrong is NOT the interesting part of the story.