Though she's a snake, Miia had never shown the classic long tongue we'd come to expect from her race. That is, until chapter six. It's really jarring, especially since it's 1, a human tongue, and 2, evident that at this point, Miia is not exactly... herself.
Chapter 12 can come across as this initially, until it's revealed that the women aren't actually as helpless as they first appear.
Chapter 13 has the girls trying to help a sick Kimihito through Suu, as she is the only one immune to cross-contamination. Their attempts are pretty much universally funny (Miia's cooking makes a triumphant return), but then they get to Papi's attempt. It manifests as Suu bearing down on Kimihito with her arms morphing into wings, parroting the argument that the rest of the girls had with Papi in a strained distorted voice, all the while having light shining out of her eyes and mouth a la NicoleBrennan◊. Jesus, Suu. This is in-universe Nightmare Fuel: Kimihito is terrified!
Rachnera is quite the intimidating sight, particularly compared to the other girls. Picture a woman with the lower body of a spider the size of a king-size bed, six eyes, and razor sharp claws, whose first full appearance is her hanging from the ceiling, glaring down at the reader in a pose suggesting she's ready to pounce. And to top it all off, the very first thing she does in her introductory chapter is mummify the villain of that chapter in silk, for reasons that are likely best left to the imagination. While it's apparent by the end of the chapter that looks can be deceiving, and she's not quite as terrifying overall as she first appears, God help you if you tick her off.
Centorea was damn nearraped by a gang (led by the one Jerkass from previous chapters) in Chapter 16. For one, it is certainly not played for laughs, and considering the rules of the program (extraspecies cannot harm humans), she was rendered totally hopeless when Kimihito is held back. Luckily, Rachnera intervenes and scares the crap out of them, but damn the manga got dark for a second.
Take another look at Kimihito. He's half a second from gouging out the eye of the guy holding him, and given how strong he's been shown to be previously, those gang members might actually be lucky Rachnee intervened.
The Reveal about the centaur culture. In essence, millennia of being a culture of Proud Warrior Race Guys and using pure strength to determine "right to breed" has resulted in centaur males degenerating into a race of hulking, super-gonky thugs, whilst the women are still beautiful warrior-maidens like Centorea and her mother as the females focused more on style and technique in fighting. It's gotten so bad that their own women can't be aroused by them, causing the centaur birthrate to plummet (as aggressive as their males can be, they know better than to actually try to force themselves on the females, for fear of the injuries she can inflict if nothing else). So, the centaur males hire human men selected by the females (the hired men are often Bishōnen types who appeal to the female centaurs tastes) to get their women "in the mood" to mate by flirting with them, being nice to them, etc... only to then pounce in and roughly copulate with the females. The way it's discussed, this is the norm for most centaur marriages, and all parties involved accept it simply because of the common belief that if a centaur has a child with a human the resulting child will be weak.
Yes, this is based on the real-world practices of horse breeding, especially in fields like horse racing. No, this does not make it any less creepy!
Downplayed in the full volume's extra pages, where Cerea's mother explains that centaurs that don't want to take part in the whole teasing cultural practice have multiple methods of avoiding it (she herself earned the right to never have sex with her "husband" by beating him in a battle).
As of Chapter 18, Kimihito's life is in danger. Someone has sent him a death threat if he marries any of the girls.
Miia almost gets raped in Chapter 19. She's out in the middle of a lake, in a small boat with her captor, and there is barely anyone around.
Chapter 20 reveals that there are traffickers that kidnap monster girls. The girls in this particular chapter are children. They were naked, too. Make of that what you will.
Chapter 22. The Dullahan. We haven't even seen her face yet and already she's terrifying. Her first true appearance was her standing across the street, staring at Kimihito hidden in shadow. A truck passes, and she's gone. Suddenly she's behind him, inches from him. She has a wickedscythe in one hand... and her head in the other. Then the scythe is around his neck as he's realizing what's happening... and the chapter ends. Chilling.
Chapter 23, Lala casually mentions that there is an entire organization hunting her. She claims to essentially be Death itself, there is something hunting her, and it's enough to make her feel rushed enough to present herself to the living.
Miia's mother. While the methods in which Miia and Kimihito deal with her are pretty silly, the fact remains that her plan is to kidnap Kimihito as a Sex Slave for her tribe. Because of this, despite how funny she is, she's still one of the series' more disturbing villains.
Chapter 35 has Mero's inevitable kidnapping by Octo. While most any reader will have seen it coming a mile away, it's still a chilling scene, especially when Mero opens the door and stares in horror as a shadow with flailing tentacles descends over her... Subverted when it turns out she was not kidnapped.
Chapter 37 reveals the frightening truth about Mero's mother. She takes Mero's tragedy fetishism and cranks it up to Monokuma levels by attempting to force tragedy on her people and her daughter, purely for the sake of doing so. She goes as far as siccing (relatively harmlessto them) piranha on the other girls and attempting to drown Kimihito just so she can force a Sadistic Choice on Mero, and the manga does not make it clear whether she has really repented. Worse is that she gets away with all of it with no real retribution, so the mermaid kingdom is left under the rule of a murderous, despair-obsessed psychopath.]]
On the other hand, she switches from Malicious to Incompetent very quickly. Though in the incompetent stage she accidentally sics a second school of piranha on Mero, traps Kirohito with rubble, and gets her smartphone (and thus the controls for her traps) destroyed by her spike trap.]] Things escalate from there, and it does seem like she's realized something by the end of it.
In Chapter 38, we cut to the interior of a hospital room, and we see something begin to materialize. Then we cut to the face of the dying Yuuhi and on the very next page, we see Lala in full on grim reaper mode, complete with Glowing Eyes of Doom, who proceeds to make Yuuhi an offer that's phrased like a Deal with the Devil. If you had forgotten how unnerving Lala can be with how little we've seen of her in recent times, this will remind you!
In Chapter 40, Doppel confronts the Orc Leader, who just tried to rape her. She then proceeded to transform into an indescribably horrific abomination seen only in silhouette.
In Chapter 44, we are introduced to Killa the Killer Bee. If her name isn't any enough of an indicator that she's dangerous, she's based off the Japanese Giant Hornet, which anyone who knows about them will tell that their stings are extremely painful. Said stings can in severe cases leave wounds the size of bullet holes. Now imagine that coming from a human-sized being. She also has a considerably less human-looking face, with red compound eyes, antennae, and a lower jaw covered in a carapace akin to the hornet's mandibles. Not only that but she can command an army of hornets to attack. And true to her species, she's very aggressive, stopping at nothing to capture Kimihito and only being thwarted by getting trapped in a sauna.
In Chapter 45, as Kimihito and Centorea interrogate Killa about the other two illegal extraspecies, she mentions that they're even more dangerous to the point that she doesn't even want to be near them. Considering that she's based off one of the most dangerous animals in the world, that's saying something.
Of course, as long time readers might expect, the other two extraspecies end up not being as dangerous as rumored (one just produces spores that cause shared hallucinations, the other is dangerous only because she is being possessed by someone dangerous).
Chapter 56 has a moment from Kimihito of all people: upon hearing the words 'milking time' on a farm staffed by Minotaurs and Fauns, he panics and his mind immediately goes 'worst-case scenario': in this case 'worst-case scenario' means sudden dark turn into horrifying factory-farm/sex slave conditions for the aforementioned liminals. The Imagine Spot gives us imagery like something out of the foulest Hentai doujin. Of course, it doesn't turn out to be that way, but HOLY HELL that'll stick with the reader for a while.
Chapter 58, which focuses on the satyrs of the Lily Farm. They take the All Women Are LustfulUp to Eleven and beyond regarding how sexually aggressive they are in pursuing Kimihito for some private time, to the point where one of them almost manage to rape him in his sleep one morning. The faces they make rivals the page picture above in how unsettling they can be, natural goat eyes included.
The fact that they move in packs like wolves or velociraptors whenever they approach him cranks the horror about them even further.
Chapter 69, which introduced an actual Nightmare monstergirl. While many of the nightmares she gave Cerea were silly, embarrassing, or sexy, she ended up having so many that she started to wonder whether or not she was awake. She was bombarded by so many nightmares she lost track of reality... And the nightmare Kimihito had at the beginning was rather creepy too...
In Episode 3, Miia, Cera and Papi all get a temporary demonic makeover whilst fighting each other. The resulting images are downright nightmarish, especially coming from the previously-dignified Centorea or the cute and childish Papi, and the hellish screeching that accompanies them doesn't help in the slightest.
Although they probably wouldn't mean to harm Kimihito due to the fact that the incident occurs during The Full Moon, the fact that they (Miia, Cerea, and Papi) lose control over their strength would mean a rather fatal intercourse. As the realization slowly hits him (and the viewer) in the anime, it brings a rather jarring tonal shift. Initially, the girls trying to seduce Kimihito is played for titillation as one may expect, but then Miia crushes three or four steel bedposts in her coils without even noticing, Kimihito nearly gets his face torn off by Papi's butcher-knife talons and Centoria smashes a table like an insect beneath her hoof, and the situation starts to look unnervingly...slasher movie. The girls even get a couple of scenes where they're hidden in shadow and manifest glowingred eyes.
Episode 11, while Miia getting frightened by Lala is Played for Laughs, ends with a powerful Mood Whiplash. In a blood-red sunset, Lala just told everyone in the house that Kimihito is going to die soon, and the serene atmosphere is replaced with fast cuts of Darling and the girls' close-ups, their faces (done more realistically than usual) frozen in shock and terror. Everything is also covered in a reddish hue. It's unnerving to say the least. The dullahan imagery throughout the episode is also quite creepy.