The Mutants appear as distinctly humanoid beings, with the added scarification and distorted features characteristic of other common mutants in fiction. Their disfigurement is suggested to be the result of exposure to nuclear fallout, radiation poisoning or viral infection therein. This is mainly taking into account that they appear to be wearing contamination suits made for nuclear testing. Their mutation has significantly altered their internal biology, changing their blood green and allowing them to generate and vomit a corrosive acid.
Tampering with the Gas Mask and Tonics summons their presence.
They are first seen in their cube cell waiting to be released.
Then they're seen during the Purge on a monitor as three of them hold a victim down while another vomits acid into the captive's mouth. Later, the Mutant in Hospital Gown, who was waiting in the cube cell with the other Mutants, attempts to kill Marty before being shot in the head.
In the novelization, the Mutants take the place of the Zombies during the first wave of the purge.
They are a reference to generic mutants in film such as the 2006 remake of The Hills Have Eyes, the Mountain Men in Wrong Turn, and the mutants from Chernobyl Diaries. They bear similar characteristics, however, to the Rage Virus based zombies such as the ones from 28 Days Later, Signal and the ones infected with the the Sicko Virus from Planet Terror. Some infected within the Left 4 Dead universe also wear contamination suits simmilar to these. The alien slug-controlled undead from Slither also had a corrosive spit they would use on their victims.