There are two common quirks I keep hearing about dragon anatomy and biochemistry.
The first is that dragons use lifting gases to keep aloft and breathe fire, both of which are just plain dumb. First, the breath weapon would be extremely dangerous (to the dragon) and have a very short range because hydrogen is less dense than nitrogen.
Second, lifting force per unit volume in air is ~10N per cubic METER, IF you have a PERFECT VACUUM. Sure, using hydrogen instead still gets you very close to that value, but it's still insanely small.
The second is a bit more interesting. Bombardier beetles and their unique natural defense mechanism is often cited as a way for dragons to breathe fire (incorrect).
Yours truly also knows that this mechanism is seemingly an irreducible complexity, a fact I love to throw at the head of "No beast in nature has four legs and wings" gremlins who can't even finish a book, let alone know what the Dunning–Kruger effect is or how evolution actually works.
Anyway, you probably know how the mechanism works, you mix hydroquinone, hydrogen peroxide, and some enzymes together, then the generated gas propels a ~100 °C hot liquid forward and also closes off the valves to the reservoirs of the beetle to prevent an unwanted explosion.
Now, the problem is that dragons are bigger, at around horse-sized, and use their mouths. So, I doubt you could just copy-paste the original design and expect it to work. Especially since the distances involved are different as well.
But how would the defense mechanism work to accommodate these changes?