Actually, contrary to this, there are actually a lot of different conceptions of dragons. The most famous distinction is that between Oriental and Occidental dragons, the former usually being considered good (although sometimes temperamental) and the latter evil. They're generally portrayed as something like snakes with legs and sometimes wings, but their size can vary greatly (Wikipedia says that popular culture portrays dragons as much bigger than the ancient legends usually did), as can their other features. Chinese dragons, according to various sources, have parts resembling those of crocodiles, deer, camels, oxen, fish, tigers, and probably just about every other animal that lives in that area. The wings of European dragons resemble those of bats, and Catalan tales sometimes give them leonine or bovine heads. The dragons carved into Viking ships have been described as having canine facial features, but I've always thought they looked somewhat like horses. Some ancient sources make dragons into constrictors, while others hunt while their claws and teeth, and of course some breathe fire. With that in mind, I'm interested in when the idea of dragons breathing substances other than fire (ice, lightning, steam, poison gas, etc.) originated. Were there any old dragon legends that gave the creatures breath weapons other than fire, or was that a more recent idea? I've seen mention of a few stories in which the dragons had the power to turn people to stone (like Medusa or a basilisk), and I think some of them might have been poisonous, but what about, say, frost dragons? Actually, now that I think of it, the dragon in Revelation shot water out of his mouth, which could possibly relate to how Asian cultures often saw dragons as bringers of rain. The concept of a dragon only being vulnerable in one area (mentioned in Bender's Game) seems to be a pretty old one. The Teutonic hero Siegfried (sometimes known as Sigurd) attacks Fafnir in dragon form from underneath. Beowulf's companion Wiglaf stabs the dragon that dealt a mortal wound to his master in the abdomen. This idea is incorporated into Tolkien's The Hobbit, although Smaug's weak spot is only one small patch (shades of Achilles' heel).
So did dragon stories spring up in so many different parts of the world because they're a product of the collective unconscious? Well, maybe, but considered the great variety of mythological creatures that get lumped under the dragon umbrella, I think it might be more of a case where, when people from one part of the world heard stories of reptilian monsters from other places, they thought, "Hey, we have some animals like that in OUR mythology, too!" Eventually, the different descriptions started to merge together, eventually becoming what we now consider to be a dragon. Of course, that's really just a guess on my part, but it would make sense. It would be pretty cool if dragon stories were actually based on accounts of real animals, but I don't think that's too likely. :P
More dragon posts will be coming soon, including one about video games, and another featuring the dragons of Oz.