The Endoskeleton is an NPC found in the Backstage in Five Nights at Freddy's, and in a few select rooms in Five Nights at Freddy's 2 as an easter egg.

It is what is found in the inside of an animatronic suit. The Animatronics are trying to kill Mike Schmidt because they think Mike Schmidt is an Endoskeleton without it's suit on, which is aganist the rules.