''Series/TheXFiles'' was a nine-season LongRunner, and monsters were the show's thing. Some of these were genetic mutants who acted on predatory instincts and followed their biological needs. However, several of them crossed into the CompleteMonster territory. Often these cases were surprisingly light on the paranormal stuff, making them [[{{Mundanger}} fairly mundane and all the more frightening]].

* The Cigarette Smoking Man, also known as the Cancer Man, is Agent Mulder and Agent Scully's main antagonist, ''the'' villain of the MythArc, and the only CompleteMonster who's not a MonsterOfTheWeek. He's associated with the grand government conspiracy Mulder and Scully are trying to uncover. While some of the conspirators are {{Well Intentioned Extremist}}s, Cancer Man doesn't fit the bill. Whenever it would appear that he showed compassion or altruism, he in fact tried to manipulate people or played evil tricks on them. He had numerous means to neutralize his opponents, but he often chose especially cruel ways. For instance, he tried to burn Mulder alive and let Scully be abducted and experimented on, which gave her cancer and made her infertile. He had his ex-wife and mother of his son turned into an alien-human hybrid. He attempted to murder his own son in cold blood; said son survived, and Cigarette Smoking Man had him experimented on in unspeakable ways. He ''did'' save the Agents' lives a couple of times, or at least he didn't interfere with their attempts of saving each other, but he admitted he did it because it's satisfying to see their spirits crushed. Although he claims he wants to atone in "En Ami", it's clear doesn't want it enough to actually change and he never feels guilty.
* Cecil L'Ively from season 1's "[[Recap/TheXFilesS01E12Fire Fire]]" is a {{pyromaniac}} who can [[PlayingWithFire ignite things with his mind]] and who himself cannot be hurt by fire at all. He's a creepy stalker and pursues women in powerful positions, [[ManOnFire burning their husbands and other family members]], [[WouldHurtAChild including children]], to death. He seems to [[{{Sadist}} genuinely enjoy torturing people]] and shows no remorse.
* [[TheyLookJustLikeEveryoneElse Donald "Donnie" Pfaster]] from season 2's "[[Recap/TheXFilesS02E13Irresistible Irresistible]]" and season 7's "[[Recap/TheXFilesS07E07Orison Orison]]" is a [[ILoveTheDead necrophiliac]] who [[CreepySouvenir collects hair and fingers]] of his female victims after he's done with them. His fetish escalates into him becoming a SerialKiller. He has some kind of twisted, sick affection for his victims which is utterly scary. He's a horribly menacing killer who shows no regrets for his deeds, looking surprised when he's caught, as if it was his right to treat people as he pleases.
* Colonel Jacob Wharton from season 2's "[[Recap/TheXFilesS02E15FreshBones Fresh Bones]]" is a {{sociopath}} who tortures people from Haiti who are held at an immigrant camp he's in charge of. It's revealed that he's responsible for several deaths: a small boy, a Haitian prisoner who knew voodoo and several of his soldiers that tried to stop him. He thought it's good to kill emigrants, rather than sending them back home.
* [[MadDoctor Dr. Jack Franklyn]] (real name: Dr. Clifford Cox) from season 4's "[[Recap/TheXFilesS04E06Sanguinarium Sanguinarium]]" is a [[DeadlyDoctor demonic doctor]] and a [[VainSorcerer narcissistic warlock]] who makes {{human sacrifice}}s by taking over the minds of various surgeons and having them kill patients in horrific ways (draining a patient's blood via liposuction, nearly cutting a patient's head off with a laser, and melting a patient's face off) so that he can retain his youth and looks.
* [[GunmanWithThreeNames John Lee Roche]] from season 4's "[[Recap/TheXFilesS04E10PaperHearts Paper Hearts]]" is an utterly evil [[WouldHurtAChild child molester]] and SerialKiller who killed 16 little girls. He [[ManipulativeBastard manipulated their parents]] to trust him, and then came back and kidnapped the girls at night. He plays sick mind games with Agent Mulder and tries to convince him that he took his little sister Samantha (whose disappearance was not explained), even though it's not true. He manages to escape from prison during the re-opened investigation and tries to take another girl. When Mulder catches them, he nearly shoots her. He doesn't regret his deeds at all and would continue if he could.