I'm gonna assume then that this entire facility was similar to Biohazard/Resident Evil where they experiment on both animals and humans..... and the one that was chasing you was a product of that experimentation gone wrong. It attacked all the scientists (which would explain why there's no one around and everything is in disrepair) and goes after any creature it finds. Your character may also be part of that experiment, because hey, how else can we explain that it can do platforming, puzzles and push & pull objects that're bigger in size. XD