The Flamerus Rex is a boss from Final Fantasy Mystic Quest, the Fiend of Earth and one of the Vile Four. It is found at the bottom of the Bone Dungeon, where Benjamin and Tristam challenge it to battle. After it is defeated, the Crystal of Earth reverts to normal and Tristam gives Benjamin the Elixir needed to rid Kaeli of poison and then claims the Dragon Claw from a nearby chest. Flamerus Rex is later reincarnated as the Skullrus Rex in Doom Castle.
As with all of the Vile Four, their reincarnations, and the Dark King, Flamerus Rex has four phases to the battle, depending on how much HP it has left. As it loses health, its skeleton deteriorates until it is but a pile of bones.