Teremus the Devourer isn't living so large this days. He was originally a vanilla world boss prior to his level and type change in Cataclysm. Before the introduction of the leashing mechanic for mobs, players would sometimes go to the effort of kiting Teremus to Stormwind in order to grief other players. (Teremus had an AoE health leech ability that scaled in power according the number of players and guard NPCs in combat with him.) The only way to get rid of the drake was a realm restart or a GM entering the game and killing him.
He is now a level 57-58 rare. His loot table was adjusted to reflect his reduction in difficulty.
For players who are phased to the Warlords of Draenor version of Blasted Lands, they first will need to speak with Zidormi in order to access this rare; Teremus only spawns in the pre-WoD Blasted Lands.