Monsters that still are alive in the X-Files show universe
whether its rapid aging processes to humans to monster mutants from the show, the ones that survived to live another day
the Fluke Man mutant. at the end of the episode Scully did tell Mulder that worms especially something like fluke tapeworms regenerate. So the mutant Fluke Man thing survived from his bottom half being cut in half.
the Firewalker fungus. Pretty sure some of it still exists in crevices etc in the volcano type caves