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 11 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 piss 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 shit 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, generations of being a a culture of Proud Warrior Race Guys and using strength/combat skills to determine "right to breed" has resulted in centaur males degenerating into a racing of hulking, super-gonky thugs, whilst the women are still beautiful warrior-maidens like Centorea and her mother. It's gotten so bad that their own women hate them and would rather chase cute human guys, especially bishonen types. So, the centaur males hire human guys 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 all centaur marriages. That's right, centaur culture is literally built around the concepts of netorare and spousal rape! It only gets worse when you consider the fact that "Teasers" are usually either amoral enough to go along with this whole plan for money... or else they actually like the centaur women in question, so they get to watch as the girl they like is basically raped in front of them. It's hard to say which is worse.
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!
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...
Chapter 37 reveals the frightening truth about Mero's mother. She takes Mero's tragedy fetishism and cranks it up to Junko Enoshima 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 realised anything 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 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.