Endoskeleton

Endoskeletons are the internal framework of all animatronics in Freddy Fazbear's Pizza, being a bare robotic skeleton that handles the movements of the robot while the suit and extra features help emphasise the characters.

Their importance within Five Nights at Freddy's Plus is unknown, though the likelihood is that they'll remain as a decorative spare much like the original Five Nights at Freddy's.

Appearance
Endoskeletons are skeletal robots made from steel, with thin and spindly limbs. They have square heads with two detatchable eyeballs attached to a outward plug, and a set of teeth extended from the head to connect to the suit's face to simulate mouth movements. By default, the eyes of the endoskeleton are a pearly white with no discernable pupils, however they have the ability to glow with red, pinprick pupils. The torso features a ribcage with five ribs on each side, likely to help the animatronic suit keep its shape. The limbs of endoskeletons look much like coils, and their hands are akin to that of a human skeleton with four fingers.

Trivia

 * The endoskeleton's design was inspired by the old, scrapped model of the original endoskeleton's made by Scott Cawthon appearing in FNAF 1's trailer.