The Stone Eater, even if it's small potatoes compared to other monsters. Picture this: you're trapped in an old, dark building, you can actually see rescuers looking for you but they can't see or touch you and there's this huge stone armor with a distorted voice and creepy fang-filled mouth stalking you around the corridors,savoring every single ounce of your fear as it prepares to turn you to stone to slowly consume you like a block of cheese, all while he wants you to know what's going to happen out of sheer sadism.
The first version of the painting that Reiko's father made for her. It's all devoid of lively colors and depicts Reiko like a creepy white ghost sitting in a chair, complete with a soulless emotion and a lifeless stare.
The Ayakashi by itself is already creepy, being a colossal, wart-covered sea serpent with a featureless head and baleen-like mouth, but then you see his stomach: a dark tunnel of black water filled with the many ships he devoured in many years, with gangly, ghoulish humanoids on stalk-like bodies all around and, at the bottom of the stomach, Ayakashi's true "face": a monstrous mishmash of faces, eyeballs, mouths and random bodyparts beyond two huge, gate-like jaws.
Fusuma. A colossal, floating green humanoid with an incredibly creepy grinning face who loves to attack airplanes, coils himself around them and devour all the humans on board, while also being malevolent enough to resorting to a suicidal move to take the whole plane down with him when fatally wounded. In one edition of the manga he was actually voted as the creepiest Youkai ever.
A couple of manga-only villains that appear on the way to Hokkaido definitively counts for looks and methods:
The Namahage appears to be a supernatural Serial Killer in a Hannya costume skinning his victims with a huge butcher knife: is actually a monstrous macacque monster with a Nightmare Face who's willing to kill and skin a young girl so that he can take human form to get close to his victims and devour as many of them as he likes. Cherry on the top, at the end of his arc he skins himself because he's so eager to try his new skin... only to be impaled by Ushio's spear and dry up to death.
Mikonjiki is another alien-looking sea youkai resembling a walking nightmare with a huge belly mouth, massive wisp-like tongue and an eerie doll-like face. Like Ayakashi, he traps an entire ship in his barrier and slowly consumes the souls within like candy, in Tora's words.
The Vampire, our first look at a western youkai: he's a creepy bastard with a mouth full of conical sharp fangs, sadism, tendency to Nightmare Face and the creepy predilection of victims: exclusively young women and girls.
The Kyouma was already creepy as hell back when he was a lustful, grudge-filled human: as a demon he's creepier than ever, turning into a huge, white-faced, one-eyed gorgon-like monster who can spit minions in the form of toothed, one-eyed tadpole-like beasts from its "hair". The form he takes when he kidnaps Mayuko is a grotesque dark face with the above-mentioned monster form as his tongue.
The Hiyo near Cape Erimo taking a form to fight Hinowa, namely a truly abominable head with a mass of tentacles poking out of the mouth and left ear and far too many eyeballs, three of which in the nostrils.
Satoru suffered from several Body Horror instances when he got possessed by the Hiyo. For one, these parasitic creatures protrude from his limbs and face when he tried to resist their control.
While Hakumen no Mono sports a Nightmare Face, his humanoid female avatars or disguises are more sinister looking and look like they're ripped straight from horror anime. The camera would even focus on their bizarre smiles with eyes facing oddly above:
Towako is described as a beautiful black-haired woman. However, she still has a lifeless stare, pale skin, black clothes and unkempt hair that hint at her evil nature. Once she reveals the Nightmare Face that all Hakumen avatars share, Towako has red eyes and a very creepy smile. The 2015 anime by MAPPAcaptured this perfectly.
Towako's real monstrous form as revealed in Episode 23 is an even more deformed body and face, along with sharp teeth◊. When she re-appears as one of Hakumen's tails, Towako becomes a gigantic naked woman with all that nightmare face and sharp teeth at their glory.
Hakumen no Mono later tried to impersonate the beautiful spirit Jie Mei, but the result comes off scary as expected, and looks nothing like the real deal. This "Jie Mei" shares all that pale skin, red eyes, unkempt hair and black motif just like Towako. What makes it worse, she appears in the dark to humans who are alone by themselves, like a ghost encounter trying to lure and seduce her victims. Oh, she also has Hakumen's wicked smile◊.
Nodoka's human disguise looks like a gangly Einstein because of the signature hairstyle. However, his Establishing Character Moment is a combination of nightmare and nausea. As the human victim fails to answer the question, Nodoka suddenly appears beside him, mocking how he doesn't need a brain. And with his Overly-Long Tongue, he proceeds to suck the victim's brains out. While the victim's demise is only depicted via a Shadow Discretion Shot, that shadow still ended up horribly as the victim's head also got flattened as his brain slowly got sucked out. The manga doesn't have the discrection shot and we get to see the poor man's cranium deflating and collapsing on itself as Nodoka makes a meal out of him.
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