This monster is of an extraterrestrial origin. It resembles a "face-hugger" creature and appears to exude acidic yellow blood when shot.
It is possible that an alien egg summons it.
It is first seen briefly leaping out of an elevator onto one of the security guards before killing him during the second wave of monsters in the Purge.
In the novelization of the film, the Alien Beast appears during the scene where Dana and Marty are in the elevator right after seeing the Werewolf. The inclusion of the Alien Beast in the scene was apparently supposed to carry on into the film based on behind the scenes footage. This never appeared in the final film however.