Well this is difficult because a YouTuber called "The Game Theorists" believes that Mangle caused it, and shows his opinion of evidence why.
But the 4th game shows a golden version of Freddy, named FredBear (So it's not actually Golden Freddy) that bites the child, after the 4 normal animatronics (Freddy, Bonnie, Chica, and Foxy) carry him over to FredBear, See More who then bites the child's frontal lobe.
So, I believe it is FredBear, but MatPat has a good side to his belief too, so I respect him for that.
Golden Freddy. If you watched or played the forth game, there is a scene where a child is put into Golden Freddy's mouth, and he bites down on the child. Or something like that.
But the 4th game shows a golden version of Freddy, named FredBear (So it's not actually Golden Freddy) that bites the child, after the 4 normal animatronics (Freddy, Bonnie, Chica, and Foxy) carry him over to FredBear, See More who then bites the child's frontal lobe.
So, I believe it is FredBear, but MatPat has a good side to his belief too, so I respect him for that.