Mecha Scorpion

The Mecha Scorpion a.k.a. The Killer Robot, is a monster that is first seen in the elevator cube prison while Dana and Marty are trapped. It isn't listed on the white board.

Physical Description & Attributes
An enormous yellow spider-like robot with four fast pneumatic legs and three arms; two small ones in front and two long ones in back armed with loud, squealing buzz-saws. The smaller arms are capable of shooting out a pair of grappling hooks to snag and drag humans closer to the robot, allowing it to deliver a killing blow with its buzz-saws. In the novelization, it is described as a "robot with flaming hands" as opposed to its buzz-saw type arms.

The robot is seen throughout the Purge when the elevators first open, where it launched grappling hooks to two guards and dragged them close and eviscerated them with its fast rotating buzzsaws. It is later seen on security footage scrambling after facility workers down a flight of stairs.

It is summoned by a buzz saw, hidden somewhere in the cellar.

Inspiration
In many science fiction stories (notably not the ones written by Isaac Asimov), robots are shown to be deuteragonist characters that can be allies to humans or be the enemies, depending on their programming or a possible malfunction.

Notable similar examples to the one shown in the film are AMEE from Red Planet, the "evil" reprogrammed machines from Terminator 3: Rise of the Machines, and the one in Zathura, who in itself is a tribute to b-movie sci-fi\sci-fi horror robots, such as the ones seen in Hardware, Robot Wars and Chopping Mall and the 2011 short BlinkyTM.