Here There Be Dragons: Seen on star charts which include the Resplendent Exponential Vector system, and in particular, that moon given over to the biogenesis project working on creating the mythologae the way they really ought to have been.
(They haven’t managed to actually create any dragons yet, but no-one wants to be caught by surprise when they do.)
Our Dragons Are Different: Inasmuch as the eldraeic mythological aman (“dragon”) was – if you believe “certain not-entirely-accepted parahistorical theories” – imagined in the image of the local Precursors, as were a number of other similar-looking mythologae of the known galaxy. What’s known about them is that they were about the right size and shape (from the ruins), scaly (from the fossils), near-solipsists (like the rijzh) who ended up wiping each other out through inability to cope with each other’s’ existence, and possessed of technology which, while in general not all that exotic compared to the current galactic mainstream, included some ontotechnological wonders or natural gifts that made them dangerous force-of-nature-level Reality Warpers to everything around them.
And they’re also responsible for the existence of various of today’s species (starting with the eldrae), various acts of ecopoesis and ecological modification, and contributing significantly to the Galaxy’s piles of archives and ancient, dangerous artifacts. (And, of course, tend to get other examples attributed to them because, well, they’re there, belike. Despite not being the only older-than-elder species out there.)
All other dragon-like characteristics are from these extrapolated.
(There are also the ékaláman, translated as “wyvern”, which look like small, non-fire-breathing Western dragons – reptilioid, following the bluelife hexapedal model with the mid-limbs turned into wings – but are not so much terrifying magical beasties as hard-to-kill dangerous flying predators and damned nuisances to people living in their home range, and specifically to their sheep, cattle, and – if flocking – children. If you’re going out, don’t forget your clockbow.)