fire

drferox:

Let me preface by saying that I am assuming 3.5 edition D&D rules where there are five different types of breath weapon: fire, cold, electric, acid and sonic.

Sonic: Exaggerated echolocation mechanism with an ultrasound frequency. They may have a structural adaptation in their jaw, or it’s possible they sort of croak like a frog.

Cold, Fire, Electric and Acid: All of these ‘breath weapons’ could actually be dispersed in the form of droplets that then react in air. This means the dragonborn basically has to spit or spray a line/cone of fluid, and if little cobras can do that, it’s not unreasonable. Simultaneously some form of energy is conducted along that fluid, giving the appearance of breathing fire/cold/lightning instead of spitting dangerously.

The acid is the most obvious one. Lots of organisms produce biological acids that would be suitable.

Fire could be produced by spraying
pyrophoric liquids. Diphosphane seems like a good contender, but results in this type of dragonborn needing to chow down on lots of bones if they use their breath weapon frequently. They would need to be stored in a gland, like a salivary gland, with an oily base and would make cutting the face of these creatures highly hazardous.

Cold may be generated by spraying endothermic fluid. Basically makes your dragonborn one big fire extinguisher.

Electricity is generated often enough in nature by organisms with bioelectrogenesis, like the electric eel and certain rays. Ramp up the voltage and provide a sprayed mist of highly conductive fluid droplets (some sort of salt solution) and it’s not unreasonable to hope the electrical charge crosses a dense enough droplet spray. These are mechanisms that exist in nature, just coupled together and highly exaggerated. I imagine they probably have the spitting-glands somewhere in the back of the throat or top of the neck, with the current generating organ in the nasal sinus and roof of the mouth.

It’s pushing physics a bit, but this is fantasy.