Zombies are monsters who used to be Human before they died and thru unknown means were resurrected and feast on the living. At one point in the past, Area 5? considered weaponizing them, but the project was abandoned. All zombies are stored in Level Z and one day, they were all released by Team Monster on the base. It took the combined efforts of Ginormica and Sqweep to capture them. Drools is a zombie.