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I recommend reading over the search results for "Dragon" on DRBO.

http://drbo.org/cgi-bin/s?q=dragon&b=drb&t=0

It's interesting that Daniel used pitch, fat, and hair to cause one dragon to "burst asunder".
Pitch is basically tar.

As for fire breathing, all it would require is for the creature to generate some kind of flammable liquid, and add a spark to set it off.
And the creature would probably want to make a "cloud" out of that vapor, rather than pure liquid -- which would mean less liquid would be necessary to get the effect. Think of what you can do with a mouthful of water, if you spray it out of your mouth. Take a picture of that -- it's quite a cloud.

IN FACT -- some men do this with flammable liquids and light it on fire! Ever seen fire-breathers?

It sounds crazy to talk about creatures spitting gasoline, etc. but we have creatures today that spit ACID. How do they make ACID and it doesn't hurt them?
Don't know, I didn't design or create them, but there they are!

Other than that, I don't know. All I know is that dragons are mentioned MANY times in Scripture, and ancient cultures (the most ancient, ƈhıną) mention them as a real creature. Remember the Chinese Zodiac (year of the dog, pig, monkey, DRAGON, etc.) None of the animals out of the 12 are mythical. But they include dragon among them. Interesting.

No, they have not ever existed.
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The word "dragon" is mentioned a good number of times in the Bible, but as far as I know they are never described in Scripture as breathing fire.
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The story of St. George is a myth. If you are curious about the background of that, the original Butler's Lives has a good history of where that strange idea came from. It's a little complicated and I forget the details. (No, I am not recommending the modernist version; the old, real version of Butler's Lives debunks the story of the dragon). Look up the entry for April 23.
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I suspect some of the ancient accounts of large reptiles are a faint memory of dinosaurs, but animals in general don't breathe fire (and acid is not fire, nor does it look like it), so I see no reason to think any reptiles would either.

Christopher.  Was it 11 cubics?   18" for every cubic.  That is tall,  and that is why he was a ferry  man.