The Giant Snake is one of the monsters on the whiteboard. Internal Logistics bets on it.
The Giant Snake appears to be a mixture of king cobra, rattlesnake and anaconda/python, but substantially larger proportions.
It is possible that a snake skin summons it.
It is first seen thrashing about in its cube, trying to get out. It is released during the System Purge during the first wave. It wraps its body around one soldier and the Killer Robot kills him, while swallowing another soldier whole, all within a matter of moments.
The second wave of security guards catches a glimpse of the Giant Snake leaving the elevator area with the Dismemberment Goblins. The snake later eats a man who is flung over a railing by one of the Deadites, catching him in its mouth before he hits the ground.
While there have been many examples of Giant Snakes in horror films, the most iconic and infamous is the one popularized by the film Anaconda and its sequels. Other films with giant snakes or killer snakes include Python, Mega Snake and Snakes on a Plane to name a few.
This particular Giant Snake may also be a reference to the final form taken by the protagonist in the movie Dreamscape.