The idea that dinosaurs, or ‘dragons’ breath fire is not far fetched at all.
Historically speaking:
In the Bible the Leviathan is said to have been able to breathe fire.
(Please take into note that not only is the Bible a ‘religious book’, but is a useful historical record that has not in any single point, been disproved)
And why are there no stories of fire breathing birds, or even tigers, that is because these stories have some truth in them.
Scientifically/Evidence wise:
The Bombardier Beetle can basically ‘breath fire’. Check it out.
Also many fossilized dinosaur skulls have unexplained empty passages.
The use of these passages is unknown. Some people believe that those
passages “act as “gas tanks” for the combustible mixture used to breathe fire.”
Back on topic:
I do not believe that aliens exist, not in this universe at least.
Cheers!