Deonyte hides in undersea locations with minimal lighting, masquerading as a simple Omanyte. By the time anything drawing near realizes its mistake, it's too late; Deonyte's many tentacles will grab and drain the life from its latest victim.
One of the ancient and long-since-extinct Pokémon that have been regenerated from fossils by humans. A Pokémon that mutated from an extraterrestrial virus exposed to a laser beam.