FNAF and FNAF 2 Theories, facts and rumors (or hoaxes)
There were 5 kids
Lots of people have thought that there were only four kids who got murdered in the restaurant, but actually there were 5 kids. You might be asking, "but duuuuuuude there are only four animatronics", well your bloody wrong, have you forgotten about Golden Freddy?
In the second game, in the mini-game "Give Gifts Give Life" when you give life to the fourth kid, if you look closely at the center (before the Golden Freddy jump scare) there will be a short screen that shows a fifth child.
1/9/8/7 in Custom Night
There are rumors that if you put 1/9/8/7 in the Custom night you will see a secret ending. However this annoyed Scott Cawthon (Game Creator) so whenever you put 1/9/8/7 in the Custom Night you will be immediately killed by Golden Freddy.
Foxy did not cause The Bite of '87
In the first game when you are killed by Foxy, if you look closely it looks like Foxy closed his jaw then open it again. So he actually have control over his jaw, so he's not responsible for the bite of '87? But this could also be just some technical mistake Scott Cawthon made.
The Puppet
The Puppet/ Marionette is different than the others, he has supernatural abilities, for example he probably made the 5 animatronics (Freddy, Foxy, Chica, Bonnie and G. Freddy). No the Purple guy did not stuff them inside the suit, the officers caught him and arrested him when he left the room, then the Marionette came and stuff them inside the suits.
Why the Puppet stuffs them inside the suits
As you see the child from the mini-game "Give Cakes To The Children" the child that got killed is the Marionette, He turned into The Puppet when he was a child so he has a premature brain so he probably thinks every adult is the Purple Guy, he wants to have his revenge so he stuffs the 5 kids inside the suits to kill the Purple Guy
Golden Freddy's suit
Another popular theory is that Phone Guy was stuffed into the Golden Freddy suit. This seems plausible, as when Golden Freddy appears in The Office, he takes the position of a slumped over corpse. Adding to it, an endoskeleton cannot be seen inside Golden Freddy, especially since he has no eyes. This also lines up with the fact that Golden Freddy is a Freddy Fazbear suit, making it a very likely candidate to be stuffed with a security guard.
6/6/6/6 In Custom Night
It was rumored that entering either 0/6/6/6 or 6/6/6/6 on the Custom Night screen would enable the Kitchen camera. This has since been proven false. No images files for the Kitchen exist within the game.
Sparky The Dog
Sparky the Dog was a fan-made animatronic that was stated to appear on the Backstage camera. He was created by Tumblr users kodiabear and nyugen. He was described as a dog-like animatronic with a missing arm. Some claimed he was "non-violent" and would not attack the player, only occasionally appearing in the doorway to Backstage. He is completely fan-made and is not in the game, as no files for him exist.
Freddy caused the Bite of '87
Still other fans speculate that Freddy himself was responsible for the Bite. On the posted rules in the East Hall in the original game, one of the rules listed states, "Don't touch Freddy." This rule may have been enacted after the Bite; however it could also be because the killer disguised as Freddy to kill the children, or due to an incident not mentioned in the games.
There are also two instances of faint handprints or hand-shaped impressions on Freddy's face, implying that someone may have been struggling against him in some form or fashion. It could also be Phone Guy's print, because on Night 4, Freddy's song plays when Phone Guy dies, so Phone Guy could have tried to defend himself from the animatronics.
However, similar to Foxy, Bonnie, and Chica, Freddy was likely in Parts/Services during 1987, possibly ruling him out as well.
The second game also seems to disprove this by hinting that incidents which had occurred in either the first restaurant (Fredbear's Family Diner referenced in the second game) or the third (the restaurant of the original game) are responsible for the handprints.
Mangle caused the Bite of '87
Another popular theory amongst fans is that The Mangle was the cause of The Bite, as its jump-scare involves it swinging from the ceiling of The Office with its jaws open. Upon inspection, it seems that Mangle attacks the upper area of Jeremy's head, where the frontal lobe is located. This further strengthens the possibility that The Mangle administrated the Bite of '87 in a similar fashion to its death screen.
It is also known that this animatronic is dismantled daily by the kids and that the staff eventually just left it as a "take apart and put back together" attraction, which could have led it to either hate children, see a child as a possible threat, or otherwise become unstable.
The Killer
The game doesn't confirm whether the man convicted for the incident was actually the murderer, or if kids were stuffed into the animatronics. Since the animatronics attempt to kill other people in a similar manner, some fans believe that the animatronics were the real murderers.
Others believe that it was a homicidal employee (as they would have access to the costumes and the animatronics' AI controls), though the newspaper says nothing as to whether the killer was an employee. Most fan theories agree that The Purple Man from the Death Mini-Games is the true murderer involved in this incident, as he is usually present with the imagery of deceased children.
A variation of two certain theories speculates that the murderer may have been a previous night guard, which would explain why the animatronics are so determined to reach (and supposedly Phone Mike Schmidt Guy before him, as heard during Night 4's phone call). Since the killings may have occurred after-hours, this theory raises the possibility that the victims haunting the animatronics want revenge but can only remember for certain that their killer was a night guard (This is leading towards the other theory of where fans believe that the Phone Guy is stuffed into Golden Freddy).