Warning: SPOILERS ahead for Army of the Dead.
Army of the Dead's battle scenes reveal that some of the zombies in Las Vegas are actually robot zombies - but who created them, and why? With Netflix planning to expand Zack Snyder's universe of the undead into prequels and spinoffs, the explanation for the robot zombies is being saved for one of these upcoming projects.
The robot zombies in Army of the Dead are distinguishable from the real zombies by their eyes, which sometimes glow blue. Two of the robot zombies can be seen in the crowd when Zeus is mourning the loss of his zombie queen and her unborn child. One robot zombie is shot by Guzman in the basement level of the casino, and another is shot by Scott on the main casino floor. Other than their eyes, the robot zombies are identical to the rest of the alpha zombies until they are killed, at which point their skulls split open to reveal metal skulls and wiring instead of blood and bone.
Snyder acknowledged the presence of the robot zombies in a Netflix Q&A, saying: "If you pay close attention, there's a number of zombies that are clearly not zombies. You see normal zombies and then you see some robot zombies." The presence of the robot zombies, like the exact origin of the other zombies, is not revealed in Army of the Dead. However, Zeus is being transported from Area 51 when he's accidentally set free, and two UFOs are seen flying away at the start of the movie, suggesting an alien origin for the zombie virus. Snyder went on to tease a couple of potential explanations for the robot zombies: "Are they monitors that the government has placed among the zombies to monitor them? Are they technology from the other world? What's happening there?"
These questions will likely be answered in the upcoming Netflix animated prequel series Army of the Dead: Lost Vegas, which is set during the early days of the Las Vegas zombie outbreak. A snapshot of these times is shown via the opening credits sequence of Army of the Dead, and several cast members from the movie are reprising their roles for Lost Vegas. The series can be expected to elaborate on some of the characters seen in the opening montage, like the team member who unfortunately gets crushed under a storage crate. It will also offer more details of the government's initial response to the zombie crisis - and explain exactly where the robot zombies came from.
Interestingly, the neon blue color of the robot zombies' eyes is also seen in some of the organic zombies. The fetus that Zeus pulls from the zombie queen's womb glows blue for a few moments before dying, and when Zeus himself is shot in the head the resulting splatter of gore is laced with blue. It's possible that the zombie virus is actually technological in nature - perhaps comprised of nanites - and the robot zombies are another experiment created using the same technology.
Though they're only a background detail in Army of the Dead, the robot zombies could play a bigger role in a potential movie sequel or spinoff. For example, an army of robot zombies could be sent in to fight an army of regular zombies in Army of the Dead 2, or the robot zombies could rebel against their human creators (assuming they were made by humans) and join forces with their organic zombie brethren. With robot zombies, the possibilities are limitless.