Athasian dragons are a species of reptiles native to Athas that are created when a powerful sorceror uses their powers to absorb enough magic from the natural world or from captured slaves to evolve into a new form. The process takes several stages, the first being the larval dragon, the form the sorceror evolves into when they complete the initial ceremony of transformation. The new dragon then progresses through a series of stages, fueled by defiler magic all the way, till they are transformed into a full-fledged dragon. Not all dragons complete their transformation, to be sure; many are left trapped in earlier stages of dragonhood. Dragons use dragon magic, which is what cuuses them to evolve into dragons in the first place,, and speak the common tongue if they were originally human, elven if they were originally elves, and so on and so forth.
DARK SUN: THE GAME MANUAL (book by Jeff grobin, section on dragons)