King Thornclaw may be an earth-cosmic hybrid
Hybrid dragons are relatively rare dragons created when two dragons of different species mate and have offspring which retain aspects of both the parents' physical appearances and properties. For example, wind dragons mated with light dragons to create today's breeze dragons. For another, a polluted Cloud Dragon mated with a Water Dragon to produce an Acid Dragon.
Unusual hybrids
In a very rare scenario, two very different elemental dragon species mate to make an elemental hybrid. For example, if a fire dragon and a water dragon mate and produce offspring, an elemental hybrid with power over both elements can be created. This is extremely rare and there have only been a few confirmed cases. The powerful King Thornclaw may be a rare hybrid of an earth dragon and a cosmic dragon.
Normally an elemental hybrid dies quickly because the two opposing elements struggle for supremacy inside of it, weakening it to the point of death.