Deviate Faerie Dragons are hostile level 19 elite rare mob faerie dragons found in the Wailing Caverns. Only one of them can be found during a visit to the dungeon however, and there is a high chance none will spawn at all.
As a rare spawn, this mob will not always be in the instance. Some estimates place his spawn rate at between 5-10%. He is a solitary mob, and is always located on the side heading towards Lord Serpentis.