Note: Big FNaF nerd rant.

The biggest argument used for the Mimic not being Burntrap is they have different endos.

Ignoring the metal claws and the fact that the Mimic has the ability to not only repair itself using old parts but also change its design to fit into any suit that’s available, this isn’t really a good argument considering the other characters in Ruin. Roxanne Wolf has a completely different endo (not just the head), Freddy’s shattered design has new and old parts, and Candy Cadet’s design changed in Ruin with no real reason.

Another argument is that MXES is used to keep the Mimic sealed away and the MXES machine looks old, and therefore the Mimic was stuck in the room by the time Burntrap existed. Although it is true that MXES was most likely older than we first expected, we do know that the Mimic was locked away after the ending of Security Breach at the very least. We know this because we literally see Gregory had a previous encounter with the Mimic after SB, therefore meaning that the Mimic wasn’t sealed away until Gregory and Vanessa were set free from mimic1. Therefore the Mimic wasn’t sealed away when Burntrap existed, and they could still be the same character.

You also need to think logically about this. Burntrap is a rabbit animatronic that can distort electrics, is trapped in a underground pizzeria, and is connected to springlocks. The Mimic we see in the epilogues of the books is a rabbit animatronic that can distort electrics, is trapped in a underground pizzeria, and is connected to springlocks. It just makes logical sense if the Mimic and Burntrap are the same character.