The Mimic can be considered to be William Afton’s soul transferred to an animatronic, known as Springtrap that’s had the springlock suit break apart. Springtrap, after being burned, becomes Burntrap. Burntrap, after being left alone for a while and decaying to its endoskeleton, becomes the Mimic. This is one possibility.
This is supported by the fact that the model for the Mimic is based on Burntrap. While it doesn’t have a rabbit-like face, it does fit into the Burntrap suit perfectly.
An alternate possability is that The Mimic in Security Breach is the endoskeleton of Circus Baby. It’s theorized that Circus Baby was a springlock suit and that there was an endoskeleton inside. This is why the only time you see Baby in Sister Location, she’s in the same position as Golden Freddy, another springlock suit.
The Circus Baby springlock suit was destroyed in the fire but the endoskeleton, aka The Mimic, inside was not. This theory is based on the fact that many of the non-princess quest endings are canonically discarded as comic book drawings, likely by Gregory.
William Afton Primer
William Afton co-founded Fazbear Entertainment with Henry Emily. William Afton is the series main antagonist and a serial killer, who ultimately was the cause of many of the various deaths in the franchise and why many of the suits are haunted. For instance, Afton kills Charlotte, Henry’s daughter and his own daughter, Elizabeth, gets grabbed by Circus Baby. Micheal Afton, his son, is the protagonist of Sister Location and is scooped, taken over by Ennard (who is not the Mimic).
Charlotte, or Charlie, becomes the puppet and places the vengeful souls of Cassidy, Fritz, Gabriel, Jeremy, and Susie in the core animatronics (Freddy, Chica, Bonnie, Foxy and Golden Freddy).
The many events of various games later, William in the 90’s rolls into an abandoned Freddy’s restaurant and gets chased by the spirits. Inside the safe room he puts on the springlock Bonnie costume then proceeds to laugh at the spirits, which in turn causes the springlock suit’s springs to buckle causing the internal endoskeleton to release crushing William.
Springtrap and Springlock Suits
There’s a mode A and mode B of most rides at Disney. The B mode is what happens when an animatronic or set piece breaks and needs to be repaired. A good example of this is the Star Wars ride, where if the Kylo Ren animatronic is down then you simply get him visiting out of a viewport instead so that the ride can continue.
Similarly, Fazbear had the springlock suits. The endoskeleton could be compressed into the suit, using spring locks, allowing someone to carefully wear the suit and take on the actions of the animatronic. Afton, inside of Spring Bonnie, is crushed as the endoskeleton uncompressed and he dies a slow death much in the same way many of his victims died.
This creates Springtrap, the antagonist of Five Nights at Freddys 3. This is William Afton inside of a springlock Bonnie suit, known officially as Springtrap. FNAF3 is when the lore of the game really started picking up and a lot of the series media sort of branches out from what FNAF3 expands on.
Scraptrap – A Quick Word
There’s Scraptrap. He’s missing an arm. Best to just assume the model isn’t canon. Scraptrap is the repaired remains of Springtrap.
Burntrap & Freddy Fazbear’s Pizza Place
In Freddy Fazbear’s Pizzeria Simulator Henry Emily hires a franchisee, who in the canon ending, lures all the animatronics into a sealed off room with Henry. This includes Scraptrap, Molten Freddy, Scrap Baby and Lefty (the Puppet) along with the franchisee and Henry. Fire comes up out of the floor and burns everyone, except of course William whose endoskeleton and bits of his remains stay intact and form Burntrap. The remains of the others form the amalgamation.
Burntrap remains sealed in the basement, controlling things via the Glitchtrap virus (a scanned version of William himself inserted into the code of the Pizzaplex animatronics / system). There’s a non-canonical (now) ending where you can fight Burntrap and escape the Pizzaplex with Freddy Fazbear. Ruin changes that.
In the Redemption Ending, or Princess Quest Ending, you finish Princess Quest 3 and Gregory sees the S.T.A.F.F. bots have malfunctioned. The Vanny outfit is left on the ground. Gregory leaves the front with Freddy’s head, which is still active. The post-credits scene shows Gregory, Vanessa and the Freddy Head happy. This is now considered canon for the DLC Ruin to occur.
Burntrap to Mimic
Which means Burntrap is locked away in the basement and continues to decay. You find Burntrap in Ruin’s ending as the Mimic. The Mimic is Burntrap without any of his remaining outer bits, is prone to wearing costumes (from the books and what we learn in the game from finding the mascot outfits) and can mimic other’s voices.
Circus Baby as the Mimic
One thing about Circus Baby is that she’s highly deceptive. You’ve got a Burntrap arm on the mimic, but it’s not a 1:1 comparison with the face. It does, however, look like it would fit right on to Circus Baby if Circus Baby was a springlock suit.
That’s likely where the mimic originated and how Circus Baby was able to escape not only the scooper but the incinerator.
Glitchtrap is the virus that is inserted into the Pizzaplex systems when a board is scanned in at some point. Glitchtrap is defeated in the Princess Quest ending and is returned as the M.X.E.S. Security Program. Vanny or Vanessa locked Burntrap / the Mimic away using it, likely after the Princess Quest ending where they’re freed and able to go back and use the code against William Afton to seal him away.
In the elevator / neutral ending of FNAF: Ruin, Gregory mentions he’s nowhere near the Pizzaplex and all he has is a map that a “friend” (like Vanessa) gave him. Then somehow, Gregory says he cut the rope (which wouldn’t have been possible since Gregory would have had to have been following behind Cassie the entire time). It’s likely in this ending the Mimic cut the rope and survived. In the scooped ending, the Mimic doesn’t survive.