King Thornclaw may be an earth-cosmic hybrid dragon.
Earth dragons (Draco terram) are wise, powerful and gentle beings. They are normally quite friendly to humans and are one of very few dragon species to allow select humans to ride them. Earth dragons are also called life dragons, plant dragons or thorn dragons.
History
Earth dragons originated in Spain, Siberia and England, where the very first of their kind appeared. One of these was their current ruler, King Thornclaw, who is reputedly one of the most powerful dragons in existence. They were domesticated by knights who thought they might make good stallions to replace horses, which paled in comparison to the dragons' might. However, the plan did not work out, so earth dragons never became permanent mounts, although they still allow very special human riders. When a human and a dragon bond, the human becomes known as a dragon friend and is able to ride earth dragons safely. This is probably where the legend of the dragon riders comes from.
Appearance
Most earth dragons have large, leafy wings and thorns growing from their claws and bodies. They have a special gaseous breath weapon of poisonous carbon monoxide. This can knock out other dragons, though it doesn't kill them, since earth dragons do not believe in killing their enemies unless necessary. They also have green, brown or tan scales, well suited for living and hiding in their forest homes, not to mention concealing themselves from predators. Earth dragons possess a space on their back free from spines or sharp edges which allows a humanoid being to comfortably use the creature as a mount.
A rare subspecies
A rare subspecies of earth dragon with multiple limbs
There is an exceptionally rare - not to mention aesthetically pleasing - subspecies of earth dragon. It is unnamed and has only a few confirmed sightings, although there are plenty of unconfirmed reports. See the picture at right.
They appear to have multiple limbs and extraordinary magical abilities, along with unnatural intelligence and the power to see glimpses of the future. These dragons have been unofficially named "mystic dragons". King Thornclaw may be a mystic dragon, although he only has the four limbs typical of most earth dragons.