how do dragons fly? (Spoilers all books to end DH)

['thul]

It was most likely a dragon. Why, you would ask?

Well, Elderlings were not winged (Thamyra was/is unique on that front, sintara said so herself...), the landing dragon in the dream-vision was mistakenly called Elderling, which means it had to be identical enough to what he (fitz) had already seen (the tapestry), and the elderlings were generally humanoid, aside from minor details (elongated bodies, too-long lives, scales... just minor stuff...)

The "tongue of flame" is most likely either an artistic error, or a misconception related to the fact that fire breaks down stuff just like the acid does, but not quite as effectively...

After all, what do you call the result of strong acid attacking the skin?
You call it acid burn... The fact that dragon acid is also dispersed as a cloud, it is not very hard to turn "a cloud of acid emanating from the mouth" into "a tongue of flame", especially not when you have limited knowledge of complex science and chemistry, as the people in any medieval society will have...

Take also into the account that one of the main things sailors on old wooden ships fear the most is fire because it can quickly decimate such ships, then compare that to the devastation Tintaglia caused to the Chalcedan ships with her spit, you have a good answer.