20. Orell Peattie FOX Episode: "Theef" (Season 7, Episode 14)In addition to looking really creepy — with apologies to actor Billy Drago! — Orell Peattie is a hoodoo man. His use of voodoo dolls is seriously unnerving, especially when he blinds Scully by putting nails through one doll's eyes. And leaving messages in blood at every murder scene is extra disturbing. 19. Soul Eater FOX Episode: "The Gift" (Season 8, Episode 11)The soul eater isn't bad — he's just misunderstood. In fact, Mulder was actually trying to put the poor creature out of his misery when he first encountered it. That doesn't mean it's not really awful to look at, though. Don't judge a book by its cover, but do run screaming. (Sorry, buddy.) 18. Hallucinogenic Fungus FOX Episode: "Field Trip" (Season 6, Episode 21)OK, so it's mostly a really aggressive mushroom. But it does digest humans alive, while simultaneously making them majorly trip out. At one point, Mulder and Scully actually believe they've been rescued when they're actually still trapped underground, being slowly eaten by yellow goo. 17. Greg Pincus FOX Episode: "Folie à Deux" (Season 5, Episode 19)Here's the thing: The bug monster Greg Pincus transforms into isn't all that frightening. The production staff actually hated how the prosthetic suit looked. On the other hand, there is genuine horror in the way only a select few can see Pincus' true form — and those people, including Mulder, are condemned as crazy. 16. Bark Creatures FOX Episode: "Detour" (Season 5, Episode 4)There's no name for the predatory woodland monsters encountered in "Detour," but it's hard to forget their glowing red eyes and unique camouflage abilities. Mulder's suggestion that the creatures are actually descendants of human settlers adds another layer of unease. 15. Parasitic Slug FOX Episode: "Roadrunners" (Season 8, Episode 4)It's hard to believe anyone could mistake this giant slug for the second coming of Christ, but hey, sometimes The X-Files requires suspension of disbelief. Either way, the parasite's implantation into Scully is horrifying, and the episode is a great return to "Monster of the Week" storytelling following Mulder's disappearance. 14. Phyllis H. Paddock FOX Episode: "Die Hand Die Verletzt" (Season 2, Episode 14)She's a substitute teacher and the devil? And her name is Phyllis? There's something really scary about her deceptively innocent appearance, not to mention the fact that she's never caught. Instead she leaves behind an eerie message: "Goodbye. It's been nice working with you." 13. Rob Roberts FOX Episode: "Hungry" (Season 7, Episode 3)Rob Roberts is another one of those monsters who isn't really evil — he just can't control his appetite. But try explaining that when he's tearing at your flesh with his rows of razor-sharp teeth. And don't even get me started on those soulless black eyes. Pity he's actually rather cute in his human form. 12. Leonard FOX Episode: "Humbug" (Season 2, Episode 20)"Humbug" was one of the first comedic X-Files episodes, but the monster at its core is still genuinely frightening. Leonard is Lanny's twin, who can detach himself from his brother's body and burrow into new hosts. Frankly, a Fiji monster — Mulder's original theory — would have been less upsetting. 11. Anthony Tipet FOX Episode: "Via Negativa" (Season 8, Episode 7)There's nothing OK about a third eye. Especially when that third eye belongs to a deranged religious cult leader (is there any other kind?) who can kill his victims in their dreams. In the tense climax, Tipet manages to possess Doggett into almost murdering Scully with an axe. 10. Virgil Incanto FOX Episode: "2Shy" (Season 3, Episode 6)Ah, the dangers of online dating. Sometimes you meet a prince, sometimes you meet a guy who regurgitates a suffocating slime to digest your body fat. The worst thing about Virgil Incanto is that he genuinely believes he's helping the women he murders, saying, "The dead are no longer lonely." Also: free lipo. 9. Beggar Man FOX Episode: "Badlaa" (Season 8, Episode 8)This little guy shows that size isn't everything, smuggling himself into his victims and killing them from the inside out. It's really disgusting. The scene where he reaches his hand out of a corpse Scully is dissecting remains the episode's most shocking, but those squeaky wheels are unsettling, too. 8. Eve 9 and Eve 10 FOX Episode: "Eve" (Season 1, Episode 11)Let's get this out of the way first: Twins are scary. But Eve 9 and Eve 10 are actually evil, which makes them even scarier. On top of that, they appear innocent throughout much of the episode — the reveal that they both murdered their fathers is a nice reminder to "trust no one." 7. Chinga FOX Episode: "Chinga" (Season 5, Episode 10)Yes, the doll's name is Spanish for "fuck." But that's neither here nor there. What makes Chinga creepier than other evil dolls is the way she and her owner Polly compel their victims to do self-harm. As scary as it is to see a doll on a rampage, watching it force people to claw their eyes out is way worse. 6. Flukeman FOX Episode: "The Host" (Season 2, Episode 2)Is the flukeman man or fluke? Whatever it is, it left a lasting impression, as flukeman is one of The X-Files' most iconic "Monsters of the Week," despite its singular appearance in Season 2. Also, it tried to reproduce with Scully, thereby making an already gross monster all the more icky. 5. Donnie Pfaster FOX Episodes: "Irresistible" (Season 2, Episode 13) and "Orison" (Season 7, Episode 7)Sometimes the scariest monsters are the human ones, and that seemed to be the case with serial killer Donnie Pfaster, a death fetishist with a particular fixation on Scully. The eventual reveal five seasons after his introduction that Pfaster was a demon after all was just icing on the cake. 4. Robert Patrick Modell FOX Episodes: "Pusher" (Season 3, Episode 17) and "Kitsunegari" (Season 5, Episode 8)Another serial killer who appeared in two separate episodes, Robert Patrick Modell was supernatural from the get-go. Like some of the other villains on the list, he doesn't have to do any of his killing directly since he's able to force his will onto his victims, coercing them into killing themselves. 3. Leonard Betts FOX Episode: "Leonard Betts" (Season 4, Episode 12)Imagine a monster so terrifying he gets his own episode title — first and last name! Leonard Betts is a cancer-eating mutant who can regenerate severed bodied parts, and if that's not scary enough, he's behind the reveal of Scully's tumor. She survives his attack only to learn that she's dying anyway. 2. Eugene Victor Tooms FOX Episodes: "Squeeze" (Season 1, Episode 3) and "Tooms" (Season 1, Episode 21)Before he was Mr. Courtney Stodden, Doug Hutchison scared the crap out of a nation as Tooms, one of The X-Files's earliest and most notorious villains. Was it the fact that he could crawl through any hole? His agelessness? That weird thing about eating livers? Nearly two decades later, Tooms is still nightmare fodder. 1. The Peacock Family FOX Episode: "Home" (Season 4, Episode 2)No, The X-Files didn't invent the idea of homicidal inbred mutants, but man, did it tackle the trope well. "Home" was so graphic and disturbing that it was the first episode of the series to receive a viewer discretion warning. Each member of the Peacock family is horrifying, but whatever you do, DON'T LOOK UNDER THE BED.