Trimbandit's Scary Animated Zombie Groundbreaker Instructable uses two pneumatics to make a partially buried zombie jerk and pitch and seem to try to claw its way out of your lawn — it's an incredibly effective illusion, one that capitalizes on the jerking, sudden motions of the pneumatic to lend a terrifying, otherworldly vigor that makes the mannequin skinned over the pneumatics seem like the living dead.
*NOTE* I recommend starting with a low amount of pressure when testing and increasing from there until you are satisfied with the speed. The cylinders are very powerful and you don't want to rip everything apart.
You will probably want to either weight or stake down the groundbreaker to keep it from shaking across your lawn. I ran a considerable length of air hose so I could keep the compressor in the back yard, since it is rather noisy.
Scary Animated Zombie Groundbreaker [Trimbandit/Instructables]