What defines a dragon? Well, a tiny little sac filled with goo. Connected to the esophagus of a dragon is the breath sac, a small organ which all species of dragons share (which makes trying to figure out the evolutionary history of dragons kind of a nightmare).
The breath sac is filled with a slimy liquid which functions similarly to napalm, as the organ also generates magic. Whenever a dragon needs to use its breath weapon, the magic ignites the liquid, turning it into a blast of fire, ice, acid, poison, or lightning depending on the species of dragon.
Obtaining the sac serves as an excuse for dragonslayers, as many wish to harness the magical powers it could harness. The dragon here is defending itself from one of these hunters…