I'm going to try a different perspective. Perhaps the dragons exist in a thick atmosphere which is highly combustible with the addition of some biologically inert catalyst.

The dragons could fly with their large masses due to the thick atmospheric, although it would lead to higher atmospheric pressures.

The chemistry of the air the biologically inert catalyst is beyond me.

A small spark could come from high concentrations of iron in the teeth and tongue. Calcium is pretty reactive in its natural form; maybe the reaction of the dragon's saliva with its teeth creates a small flash. The saliva would be like snake venom, contained in sacs. That'd cause a lot of tooth decay. Add shark teeth growth so the teeth are replaced.

Modify the world to support dragons, then bring humans in as otherworldly visitors in advanced habitats and pressure suits. I'm not sure how to make the humans appear medieval in that circumstance, but the question didn't request the usual fantasy backdrop. Nor was any implication made that humans must be comfortably habitable.

I think most of the answers focus on bringing dragons to humans. I argue the opposite would be easier to do scientifically.

I think most of the answers focus on bringing dragons to humans. I argue the opposite would be easier to do scientifically. Modify the world to support dragons, then bring humans in as otherworldly visitors in advanced habitat modules and pressure suits. I'm not sure how to make the humans appear medieval in that circumstance, but the question didn't request the usual fantasy backdrop. Nor was any implication made that humans must be comfortably habitable. 

Perhaps the dragons exist in a thick atmosphere which is highly combustible with the addition of some biologically inert catalyst.

The dragons could fly with their large masses due to the thick atmosphere, although it would lead to higher atmospheric pressures.

The chemistry of the air and the biologically inert catalyst are beyond me.

A small spark could come from high concentrations of iron in the teeth and tongue. Calcium is pretty reactive in its elemental form; maybe the reaction of the dragon's saliva with its teeth creates a small flash. The saliva would be like snake venom, contained in sacs. That'd cause a lot of tooth decay. Add shark teeth growth so the teeth are replaced.