Mutate
Mutates are encountered only in Act 6 Chapter 2. They are first seen chasing a group of Burnouts and after Grayson shoots one of them he realizes that their blood burns through anything. Ishi states that they are "full of radioactive hemolymphs", meaning that their blood is highly acidic, melting stone and metal with ease. As hemolymph is a fluid found in the circulatory system of arthropods such as spiders and crustaceans, it is highly likely that the mutates are in fact a species of spider mutated by exposure to radioactive waste underground, just as the Burnouts and Creeps were humans affected by the by-product of the protection mechanism that shielded civilization from the radiation storms that ravage Stygia. They are quite organized and intelligent, and use kamikaze tactics to try and destroy a lift that the team uses by smashing themselves in the gears to melt through them. It is possible that they prey on Burnouts, as they are first seen chasing a group of the creatures through the underground cave system; Ishi described the Burnouts as "terrified" and moments later, their blood-splattered remains are seen on the walls of the room they fled into, implying the Mutates killed them all.