Golden Freddy

Golden Freddy (also known as Ghost Freddy, Radical Freddy, and referred to as Yellow Bear in the game's files) is an antagonist in Five Nights at Freddy's. He is a secret, golden version of Freddy Fazbear that appears in the player's office after a certain sequence of events are triggered.

Appearance
He appears as a golden/yellow version of Freddy Fazbear. He seemingly lack of eyes. He also has a dark blue colored bowtie, as opposed to Freddy's black one. However, it might be possible his bowtie is colored blue as a result of the lighting, as shown by the fact traces of blue are on his hat, as well. If you also look closely, you can see him holding a microphone. His fur is golden/yellow instead of brown, and his jaw is always hung open. His head is tilted to the side, and he is presented in a sitting, slouched position as if he were dead. Here is his appearance, and how he kills the player. Note the poster that appears in the hallway before Golden Freddy appears. Ditto, confirming game crashing, as well as appearing during The 1st Night.

Summoning
Checking Cam 2B, you will see a poster on the wall. It can switch between three different appearances: a normal picture of Freddy, a distorted version, and a close up of Golden Freddy. If the player sees the Golden Freddy poster and then switches back to the main room, regardless of whether one switches to another camera before doing so, Golden Freddy will appear.Alternatively, if the player inputs "1987" in the Custom Night set up screen, his death scene will happen instantly, closing the game. This is most likely a joke from Scott Cawthon as a result of the rumors constantly spreading about a supposed "1987" myth. See here.

Behavior
Golden Freddy will appear in the player's office after being summoned, and will cause different images to flash on screen for split seconds along with the words "IT'S ME". Golden Freddy will wait idly for a few seconds before killing the player, causing a game over. Every time he kills the player, the game will exit.Unlike all the other animatronics in the game, Golden Freddy does not have any movement or hint that he has approached and arrived. He completely disregards the doors, appearing whether they are open or closed. Unlike the other animatronics, the screen will be covered by an image of Golden Freddy's face upon attack.

Dealing With Golden Freddy
Golden Freddy will very easily kill the player if given the chance, but there is a way to prevent a game over.

Once Golden Freddy enters the office, there is a short period of time before he kills the player. The moment the player sees him, the player must stare at him for 1-2 secs, then proceed to switch to the security camera tablet, and quickly change from Cam 2B to any other cam. Doing this will cause him to disappear and prevent the player from getting a game over.

Alternatively, the player can avoid him entirely by simply not looking at the West Hall corner camera poster (Cam 2B), which will prevent the player from summoning him in the first place. However, this may prove difficult, as players may be tempted to watch this particular corner in case Bonnie is making a move toward the security office. If the player does see the poster, one can simply switch to another camera to avoid summoning him: he will only be summoned if the player immediately lowers the monitor after viewing the poster. If the player switches to another camera after seeing the poster before heading back to the office, it will not trigger his summon.

Theories

 * There is a plausible theory that Mike Schmidt might be hallucinating Golden Freddy, as Golden Freddy only exists if the player glances at said poster and returns to normal view. This is also emphasized by Golden Freddy not being a part of Night 7's A.I Customization. It doesn't even show a game over screen; the game just crashes, which supports the idea of it being a hallucination.
 * It is obvious that Phone Guy was killed and stuffed into a suit while he was recording The 4th Night's message. some speculate that the suit he was stuffed into was that of Golden Freddy's. However, upon close examination of Golden Freddy, you can make out pieces of an endoskeleton, noticeably in the space between his right hand and arm.
 * It is thought that the costume itself is an older, possibly original version of the current Freddy Fazbear costume. It is thought to be yellow/gold due to aging and deterioration.

Trivia

 * Whenever he is activated in-game (which is shown by the poster), it seems that a high pitched, jittery giggling can be heard, sounding similar to a child giggling. It is unknown if this is Golden Freddy's laugh, or something else. This laugh is actually the laugh Freddy makes while he's moving, albeit unedited, unlike Freddy's laugh.
 * When the player looks at Golden Freddy himself, a strange, distorted mumbling noise can be heard as the hallucinations flash on screen. The sound byte in the game's files is called "robotvoice". The voice, once reversed, initially sounds like gibberish, but after nine seconds in, it starts replaying parts of Night 5's phone call. Listen here.
 * When he kills Mike Schmidt, the noise heard during the close up on his face is a much slower version of the standard "death sound" heard in game (titled XSCREAM2 and XSCREAM, respectively).
 * As of Patch 1.13, setting the AI to 1-9-8-7 in Custom Night causes Golden Freddy to instantly kill the player and the game to force quit. This was likely put in by the creator to end rumours about The Bite of '87.