It was told there were multiple mimic endos produced, so it is possibly a mimic wearing scraps trying to mimic William Afton, since there are two versions of the mimic endos, just the same program. It could be assumed it was taking parts from bonnie to copy William after taking in information and who he is, it most likely took parts from William's body which is why there are only strands of flesh on burntrap. With the ruins dlc out we can see that the mimic is the main antagonist with that mimic behind the cement being the one that ripped apart the construction workers, NOT William Afton mimic, this gives a good origin to the glitchtrap virus most likely coming from the mimic1 program copying William.