Size - Drakos Arch skeleton is probably one of the “gigantic” dragons mentioned
In terms of what happened to it, it almost looks as if an Earth mage skewered it through the head and body
OR, going with the evidence from the shipwreck on the island as well, perhaps a super strong wind mage/curse user caused a hurricane that knocked the ship out of the water, and the dragon out of the sky and caused them both to crash
there are multiple sets of three chests, be more precise
ooor maybe it was just killed long ago and got buried, but then when Theos and Durza McCopyrightInfringement had their little squabble that destroyed the old world all that earth and stone pile-up fell off and the skeleton was revealed