Endoskeletons

 is the base for the inner workings of an animatronic, and may contain the actual machinations for the character. Our beloved Foxy has one inside of him. Unfortunately, you can see his endoskeleton exposed due to the severe neglect to his costume (The fur and clothes that cover him) in many of his incarnations.

The Anatomy of an Endoskeleton
It is like a humanoid skeleton made of metal that supports the costumes of each character. Endoskeletons themselves have no official gender, as they take on the gender of the animatronic costume they are supporting. The endoskeletons from the 1st and 2nd game have many differences.

Endoskeletons

No Bare Endoskeletons
According to Phone Guy it is against the rules for any Animatronic to be seen without their costume. This is one of the reasons he claims that the Animatronics will attempt to stuff Mike Schmidt into a Freddy Fazbear costume, as they may see Mike as an Endoskeleton without its costume on.

Appearances
These endoskeletons are slightly different from both games. Excluding their eyes, their teeth and their metal body, they look obviously different.

1st Game
The endoskeletons from the first game has no ears and no nose. It looked very much like a human's skeleton. Of course, it looked like blocked. It has teeth, eyes with their own eye color, four-fingered hands, three toed feet and finally, has a metal body. You can see one endoskeleton in the backstage for the 1st game. The animatronic themselves also don't stuffed the endoskeleton into a suit, why they stuff you instead is still unclear. The animatronics thought that the guard might be an endoskeleton, and since this is against the rules, they are forced to stuff the player into a suit.

In the trailer for the first game, Bonnie is seen taking off his own mask and show the player his endoskeleton. Also, when Bonnie is attacking you, as he opens his mouth widely, his endoskeleton teeth is barely shown. Chica also have an endoskeleton teeth seen when she opens her mouth from above. Freddy will show off his endoskeleton parts, as Phone Guy suggests that he is more active in the dark, and animatronics in dark places show the white pupil of endoskeleton eyes rather than their costume eyes. Freddy is mostly seen showing his endoskeleton eyes throughout the game, however his eyes are normally blue, such as when he is on stage, in dining area, in the east hall (not when he is in the corner) and when he attacks you after the power ran out. When he attacks the player before the power fails, one of his eyes is normal, another being the endoskeletons'. Foxy in the 1st game is also the most endoskeleton-like animatronic, due to his torn up and tattered appearance which might causes him to seem more furless.

2nd Game
The second game's endoskeleton design is different from the first. Additionally, it has more apparent animal ears, a more humanoid body and the white and colored eyes of character costume. (The color of the eyes varies based on which character the Endoskeleton belongs to. There's a bare endoskeleton that sometimes appears to have come out of the gift box and may even crawl through the left vent. However, as this endoskeleton is not seen anywhere else, and does not attack, it might be a hallucination.

Trivia

 * Foxy might be seen as a bare endoskeleton by the other animatronics, due to him having his much of his endoskeletal parts showing through the rips and tears of his costume. This may be why he seems to hide from the others in Pirate's Cove only to dash down the hall, and then hide again.
 * The endoskeleton sitting in the back room from the first game may belong to Golden Freddy, as this animatronic suit appears to be limp and lifeless, with no eyes.
 * The rarely seen endoskeleton from the second game might possibly be Golden Freddy's.
 * This had proven false becaause Golden Freddy seems to already have an endoskeleton in him, from his shredded ankles.
 * This Blue-Eyed endoskeleton may belong to Freddy (As it has Blue Eyes).
 * Foxy's FNaF 1 Endoskeleton has only two toes. In FNaF 2 he is seen with three toes.