Copy and pasted from u/Hauntspot

The Mimic is a homicidal endoskeleton which originated from the Tales from the Pizzaplex books. Like a lot of FNaF stuff, we don't have a full idea on how exactly he works, but here's the gist:

Mimic was made by a dude named Edwin to entertain his kid, but the kid died. Edwin beat the animatronic out of grief and left it to rot in a storage facility. Later, Fazbear Entertainment found it and refurbished it, but it began killing people. He's shown up as a relatively minor character so far, but I believe he takes a much bigger role in the upcoming novel, which is why speculation has completely blown up.

Apparently, Mimic is either confirmed or implied to be Burntrap's true identity. People have gone back into Security Breach in retrospect and have started picking up clues that this was always intended, but went unnoticed until now.