What might actually lay inside the head of one of those possessed and disturbing animatronics from the game Five Nights at Freddy's?

Well this might be something like it, with some of my own ideas and observations drawn from other things that do work; plus a little humor.
"...note the long lever arm distance from jaw hinge to hydraulic ram pinion, translating considerable pressure into jaw closure..
also the jaw rotation limiter bracket missing a pin. Result, jaw can open rather a long way, and close again with a lot of force."

crunch