Ferix

The Ferix was a monstrous, ferret-like creature who lived in a cave high in the mountains of the Isle of Mobo. It exhales acidic stecca fog. The Ferix hibernates yearly, and upon awakening it preys on the Mobo. It was defeated by a feather that Grandfather had cast a spell upon, and when Niddler tickled the Ferix with the feather it transformed into a harmless little creature and scurried away. Presumably the Ferix was put there to guard the Sixth Treasure of Rule, which was hidden in the back of its lair: after the defeat of the Ferix, the Sixth Treasure removed the stecca fog from around the island.