I believe that they did exist at one point in time not now though due to the fact if there was something that huge it would have been randomly spotted by someone by now. But yes I think that dragons did exist and it has been proven that they could have existed and that their are dinosaur fossils with deep scratches in their bone structure that is not known how they could have received them at that point in time because with how technology is now we can tell what it was that killed something.
Dinosaurs on how they were killed off was explained in two different varieties. One way is an asteroid killed them off and the other way is that the Ice Age killed them off but me with the paleontology research. I believe that they were killed off by a combination of the two. I believe that a giant asteroid did in fact crash into the planet millions of years ago but the impact isn't what killed them the sulfuric shock wave that killed off the plants is what killed the big number of them because you take the plants away from them and what happens? The herbivorous starve to death and the carnivorous end up starving to death because most of their diets revolved around fresh and live meat not already dead. So they go threw process of elimination and kill off the weaker of the carnivorous first and when the Ice Age hit at the time it did that doesn't give the reptilian bird much time to hibernate and thus kill them off in a matter of days, weeks, or even months.
So for the Dragons they too would have fallen into the same spectrum of the dinosaurs, yes they could have flown to a mountain or dig a cliff but also remember human kind was just appearing around that time and I can pretty much tell ya that they would have probably eaten any kind of meat so what ever dragons would have survived the asteroid or the ice age were more than likely killed off by early mankind.
Yes, you can say that they didn't have the proper tools or equipment for killing such a beast but there you are wrong because there are arrow heads, ax heads, and spear heads that have been found that were from all those years ago that had magnetic qualities to them which did come from an asteroid landing spot in Canada and the weapon heads have been found all over the globe.
So all in all yes Dragons could have excited at one point in time but were all killed off.
Myths come from Legends, Legends come from Stories, Stories come from Travelers. Everything has to start from somewhere and Dragons may have lived along with all other mythical creatures.