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?
Answering to the question as posed, and supplementing somewhat on @Zonetrooper 's answer, for the purposes of worldbuilding “dragon” ends where your audience believes you made it end.
Yeah, like that’s so helpful. But I feel that’s kind of the point. In Neverending Story, the story and characters are presented in a way that we are willing to believe Falkor is a dragon, we don’t need much more argument on the subject.
Monster Hunter has what IMO is one of the most interesting takes on the subject, with people having their own little in-universe arguments about what even counts as a dragon and why versus the more general surrounding concept of “wyvern” — yet, all in all, we are willing to believe that the people in the MH world are willing to believe that Kirin, a thunder pony, is a dragon, much as a giant graveyard squid is also a dragon. So we can have fish dragons, jellyfish dragons, amphibian dragons certainly, if we believe that there’s enough intrinsic weirdness to them to make them dragons.