alt Hacker NewsThat's begging the question. We don't need to look "at every kind of mythological creature that gets classified as such nowadays" from "cultures that wouldn't have recognized the concept".
One could stick on those classified as such in western culture - which is where the fantasy novel about dragons and knights and spells and the rest are based on.
And in there, dragons have quite specific characteristic and vibes, as evidenced from medieval iconography of St George to countless fantasy book covers and illustrations.
But if we do expand the set under consideration then I'd suggest they do in fact have specific things in common. Large flying carnivorous reptiles. That won't cover all the various edge cases but I think it describes the vast majority of the popular usage of the term.
That definition would imply that sufficiently large flying dinosaurs qualify as dragons. And at least personally I think I agree with that conclusion. Dragons aren't purely fantasy, they're merely extinct (and never breathed fire IRL).