Boomer

The Boomer is one of the cameo monsters from the videogame Left 4 Dead. It appears along with the Charger, Hunter, Smoker, Tank, and the Witch. The Boomer has a cameo because the directors originally planned to release a Left 4 Dead exclusive DLC maps that featured the Cabin In The Woods Cabin and Facility. This idea was eventually scrapped when the company producing the DLC went bankrupt early in the process.

Physical Description and Attributes
The Boomer is appears as a middle-aged man who has been horrifyingly mutated over time by an unknown zombie virus, causing him to develop large puss sores and mutated tissue from his body, most notably his face, neck and arms. His stomach has also expanded five times its original size, and his skin has turned a grey, fleshy color.

It is possible that a vile of Boomer Bile is what summons it, as this is the green, sticky-like substance that the Boomer pukes onto survivors in the game, and can be used as an item in Left 4 Dead 2.

Appearances
The Boomer can be seen standing idly in its cell prison, two columns over from where Marty and Dana reside during the pan out shot that reveals the assortment of monsters during their elevator descent into the depths of The Facility