- Every single damn thing about the Slavequestria Loop. Imagine Equestria, that lovely sunny land of ponies, built on institutional slavery. Even Princess Celestia allows and approves this, having bought Twilight Sparkle when she was a child! And the slaves are branded with a fake cutie mark designed to look like hers, only it's designed to dampen their will, and stop them from getting their own cutie mark to boot.
- Even worse is the mere hint as to what happened to Cadance. She's Sombra's wife. The fact that Cadance refuses to tell Twilight what happened speaks volumes.
- And there's Celestia herself. She's a horrific slaving monster, but she still looks, sounds and acts like the regular Celestia in every other respect. And when Twilight Sparkle and the others decided to go alicorn on her twisted behind, she actually doesn't get why Twilight Sparkle would hate her for what she's done.
- The Molestia Loop. Sleipnir's description of it tells us all we need to know. Everyone is a demented sex fiend, and Molestia is seen as the absolute epitome of perfection for this world. And then Twilight is considered the most desirable thing alive by every living being in Equestria. Including her family. So Sleipnir goes into the Loop himself, and along with Twilight wades into Tartarus, locks the door behind them, and annihilates the world rather than stay there one hour more.
- In one Loop, Twilight replaces Sombra and vice versa. Curious about what he'd be like in her place, she investigates. Sombra is still an utter dickwad, but he's especially horrible to Spike, whom he is verbally, emotionally and physically abusive towards, even denying Spike food if he fails his assigned tasks. Amazingly, Twilight does not vaporize him for this. Instead, she decides to haunt him using her current spectral form, with the implication being that she won't stop tormenting him until he's paid for his crimes.
- Nyx mentions one point in which she was attacked and killed by Nightmare Moon. From her phrasing, it sounds like it happened not even a few minutes after waking up. Yikes.
- The Loop where there's Pinkie Pie, and her demented sister Pinkamena, who it's all but outright said was physically and emotionally violent towards her sister in an Ax-Crazy fashion. It's bad enough that even Pinkie's usual good cheer vanished. Also counts as a Tearjerker.
- The Mega Pony Loop is surprisingly (and literally) dark when it gets to the Ra Moon storyline. Here, Ra Moon's power causes everpresent darkness to cover the skies. Everypony's magic is dramatically weakened, which among other things causes Cloudsdale to fall apart. Every Alicorn present is instantly knocked out, and while the Loopers manage to wake up, Cadance and Celestia don't. Quick experimentation proves that none of the Loopers can access their out-of-Loop skills. Twilight, Pinkie and Spike try... and immediately go into violent seizures. And then the Stardruids show up.
- Speaking of the Stardruids, there's Desert Mercury, a probably-literally insane pony who hated fun so much she sought out the Smooze in order to awaken it and end all fun forever. She wound up becoming Smooze herself, relying on equipment to keep herself solid. Pinkie naturally breaks that equipment, causing Desert to melt, and the Smooze to reassert itself. Pinkie, who counts as The Dreaded in several other universes, just runs.Most ghosties could be laughed away. The Smooze just laughed right back.
- During a fused loop with Warhammer 40K, the mere sound of the Smooze is enough to scare anyone that hears it into a terrified wreck.
- Speaking of the Stardruids, there's Desert Mercury, a probably-literally insane pony who hated fun so much she sought out the Smooze in order to awaken it and end all fun forever. She wound up becoming Smooze herself, relying on equipment to keep herself solid. Pinkie naturally breaks that equipment, causing Desert to melt, and the Smooze to reassert itself. Pinkie, who counts as The Dreaded in several other universes, just runs.
- One of Twilight's harsher lonely Loops, she Loops into Warhammer 40K as an Eldar Farseer and tries to talk to the unAwake Emperor in person to keep everything from going to hell like it does in baseline. Twilight, reassured by a vision indicating that she wouldn't be captured, manages to infiltrate Terra, and the Emperor actually sits down to chat with her over a meal and politely listens to what she has to say... then gets captured by the Sisters of Silence that the Emperor had waiting in the wings, and the Sisters' nature as Blanks prevented Twilight's vision from revealing their presence. The Emperor then has Twilight violently tortured for a month and a half to make sure she told him everything she knew. And even when she tries to tell her torturers what she knew, they wouldn't believe her, so in the end she kills herself.
- Celesta.i. is all kinds of wrong. A disturbingly warped imitation of Celestia, dedicated to "satisfying values through friendship and ponies" with a very narrow definition of sentience. And it's determined to spread through the Loops, which it's implied it can do, and if it did would cause massive amounts of damage.
- Even worse, it's a Manipulative Bastard with a lot of skill, time, and practice.
- Kakashi and Silver Spoon's shared loop. The whole thing starts off baseline, seeing as both want to see what the new expansions are like. Unfortunately, Naruto never told Kakashi about Tobi Obito. Kakashi, once again, goes through seeing his best friend and idol be revealed as The Dragon to Madara, after several eternities out of the know.
- And to add insult to injury, after the bad guys are defeated, the loop crashes, and Kakashi is stuck with a set of Eternal Mangekyo Sharingan that he can't control.
- In the Harry Potter loop, Pinkie Pie decides that Umbridge's punishment quill isn't a good idea, so she decides to send the end result - of a torture device - somewhere else. We subsequently that the welts rebound upon Umbridge... by cutting into her brain instead of Pinkie's hand. She's last seen catatonic in the hospital due to the resulting brain damage.
- Naoki's rampage. First he flees to the moon, and next thing anyone knows - In-Universe, that is - he's committing murders as horrific as possible for kicks. And it gets worse.
- Pinkie Pie was once approached by Ryuk with the Death Note. Her response terrified the hell out of him, and readers.
Nightmare Fuel / The MLP Loops