The Lurker is a large mutated frog created as an early experiment into B.O.W.'s by Dr. James Marcus, co-founder of the Umbrella Corporation. After being exposed to the T-virus, the frog grew to a large size and its skin developed into an extremely tough hide that was impervious to bullets (but still vulnerable to knife strikes, oddly enough). The creature's eyes degenerated to uselessness, but was compensated by the development of extra-sensitive hearing. Very few Lurkers were produced as research on them was discontinued since the specimens created developed no increase in intelligence and would attack and eat anything that moved, making it impossible to train them.
The few Lurkers created survived disposal and remained alive in the waters surrounding the Raccoon City Sewage Treatment Plant for years. In Resident Evil Zero, they would be briefly encountered by Rebecca Chambers and Billy Coen. The Lurkers could not be harmed by gunfire but were easily startled, prompting them to dive back into the water when attacked. Supposedly, Lurkers were spotted during the Raccoon City Incident by survivors attempting to flee the city via the sewers. The creatures were likely exterminated when Raccoon City was destroyed by nuclear bombardment.