I recently fleshed out some details on dragons as they exist in my world. I jotted down some ways in which they differ from typical fantasy dragons - and then was immediately confronted with the realization that, well, “typical” fantasy dragons don’t really exist, do they?
You have the classic, four or two legged, winged reptiles that breathe fire. Even those can differ in many ways - the ones of Tolkien’s legendarium were very highly intelligent, while those in ASoIaF are rather smart beasts, but beasts nonetheless.
Another often-seen type of dragon, the so called “eastern” dragon that can fly without wings, also almost never breathes fire. We can narrow down their similarities to the aforementioned type by the fact that both are reptilian, and both can fly - but then you have various mythological wyrms and drakes, which are very often considered dragons (such as Fáfnir or the Colchian Dragon) while not being described as able to fly at all.
This leaves us with the vague commonality of a large reptilian creature. I assume this makes sense, as the actual animal often brought up as a real-life dragon is, in essence, simply a very big lizard.
Of course, you have outlying cases like the Wheel of Time series, where Dragon refers to a specific human being. He isn’t a large reptile, and at least as far as I have gotten in the series, he can neither fly nor breathe fire. Nor does he hoard gold.
To arrive at the point, then I’d like to posit a question - what’s the furthest you feel you could stretch the word “dragon” in a fantasy setting? Is it fair to simply place Komodos in your setting and call them dragons? Are more mammalian dragons, like the Neverending Story’s Falkor, stretching the line, or could it be stretched to include even more blatantly mammalian creatures? If so, can you have fish dragons? Amphibian dragons? Crustacean dragons? Jellyfish dragons???
Where does “dragon” end?
Like most tropes, the concept of the “Dragon” is an ever-fluctuating one. Rather than one thing, it’s a whole constellation of ideas bound together by a vague common name. (In fact, the prototypical image of a “western dragon” - what we most often think of as such - very likely borrowed iconography from pre-Christian mythological imagery as well.)
But if “tropes are tools”, as the saying goes, how far can you push the concept of the dragon?
I would say the easy (perhaps unsatisfying) answer is that a dragon is no longer a dragon when your audience does not relate to it as such. That actually gives you extraordinary breadth to work with, to the point that I think there are three major points you have to hit to make something a “dragon”:
A dragon is ascribed physical properties which make it dangerous or alien. It will not share notable traits with domesticated or helpful animals.
A dragon bears supernatural power or significance beyond being a mere animal. It eclipses all “mundane” predators or powers.
A dragon is, still, not the most dangerous thing about. It can be slain by a powerful warrior, bound to a greater malicious power, or coopted by an appropriately daring individual.
A dragon is an individual or entity which metaphorically bears the traits above.