It does remind me of a programme I once saw that tried to put together a resonable case for dragons, which went something along the lines of:

A large reptile with a large capcity and very acidic stomach. The stomach acid could be used to generate hydrogen from various food sources (as a chemist this is the bit I find most troublesome).

The hydrogen could be retained in sacs withing the body effetively creating a balloon, leading to it's ability to fly. This would require the dragon to live in a cave as shelter from high winds. The wings and associated musculature would not be required to generate lift but just to steer, which might just be possible for such a large creature.

Because a large part of its volume would be hydrogen sacs (especially if it had enough to fly) attempts to kill it would be difficult as the vital organs make up a relatively small part of the animal. Also the penchant for "magical" swords being required to kill one migh be explained by the damage to ordinary weapons caused by puncturing the stomach and so being exposed to a high concentration of hydrochloric acid (HCl).

And last, and most obviously, the hydrogen could be burnt off to create the "breathing fire" legend. Again I'm not clear on the ignition procedure, or how the dragon avoids setting fire to itself.

Finally, on the death of the dragon it capcious stomach acids destroy the bulk of the carcas leaving us with no skeletons etc to confirm their previous existence.

I have also heard theories using phosphine gas (PH3) as this spontaneously combusts on contact with oxygen (O2), but this is heavier than air so the dragon really would need wings to fly.