The Nightmare Mech was a robotic, bipedal walker that was either built or found by Zim in the Nightmare version of Professor Membrane's lab. He used it to rescue Dib from Nightmare Bitters' clutches and escape from the Nightmare Skool. It was then set on autopilot to provide cover fire for the two unlikely allies as they made their escape; it was demolished by Nightmare Bitters shortly afterward.