Physical Description and Attributes
It is depicted as a normal millipede, but of abnormal size, meaning it has the ability to strangle, bite and use venom to kill or incapacitate its victims with ease. Millipedes have been on Earth for thousands of years, meaning this one in particular is either mutated in some way, or has surpassed its expected rate of growth and ignored Natural Selection, similarly to the Giant Lizard, Giant Tarantula, and Giant Centipedes.
The summoning item from the Cabin's Cellar for the Giant Millipede is unknown, but it just might be a stuffed millipede in a glass dome, when tampering with it.
The Giant Millipede is not seen during the Purge, however it can be seen in its Cube Prison, directly below the Boomer, just barely out of sight, attempting to crawl up the walls of its cell. It can also be seen up close in the behind the scenes footage of the film.
History and evolution itself could be the sole inspiration for this creature. Another possible inspiration could be from the 2005 video game Peter Jackson's King Kong, where giant millipede-like creatures attack and stalk the player throughout the game, along with other giant creatures from the dinosaur age.