Have some very large earth-mountain dragon with snow sprinkled on his back from loitering too long in the clouds.

He's loyal, steadfast, blunt, and takes everything seriously. For the Great Dragons of Cor Aethereus, they are their element and their element is them. Wherever their element is present, the dragon is present as well. The form of Terranus you see before you is the manifestation of Earth. The Great Dragons are most comfortable in their dragon forms, but they can also take the form of a Nidian easily. If it becomes necessary or if they simply wish to, they can take the form of essentially anything, so long as it is close enough to their element.