Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Night Mare Fuel: Pattycakes
The very idea of having a trusted friend kidnap you, hold you prisoner, drug you into a weakened state of mind and body, and then subjecting you to all kinds of humiliating and demeaning Age Play against your will is terrifying in and of itself. The author of this story plays the angle or everything it's worth to make the Mind Rape of Rainbow Dash a read that you will lose sleep over.
Further explaining, Dash is completely in-character in the first story, so the reader gets to watch as the spirited tomboy with dreams of becoming a Wonderbolt have her mind destroyed and turned into an incontinent Man Child.
This fic made a diaper getting used horrifying, because it means that Crazyshy's treatment is settling into Dash's mind.
The foalmula. A drug that is completely undetectable and can be put in anything. Anything. That will completely destroy your mind and make you a completely dependent Man Child. That alone is all kinds of Paranoia Fuel that will have readers eyeing their orange juice suspiciously for years to come.
Worse yet, is the dose is strong enough, the process will be permanent. Depending on the ending you get in Part Two, up to two ponies will have gotten this kind of dosage.
In the bad ending of Part Two, Scootaloo drinks the "wrong" drink and becomes so far regressed that she loses the ability to speak. At least Dash had Baby Talk.
The foalmula is a mind-altering drug that strips away inhibitions and adult, rational thought, and if in strong enough doses, can damage the brain enough that the victim is rendered infantile forever. Stew on that long enough and you can say goodbye to a good night's sleep.
Long story short, after running a gauntlet of tests that Fluttershy subjected her to, Scootaloo is given a choice. She is in one of two rooms divided by a glass window, and across from her in the other room is Dash. Before Scootaloo on her side is a lever. If she pulls towards herself, she gets gassed by the foalmula and permanently regressed. If she pushes towards Dash, then she's one who gets gassed. Well, after reading from a diary which may or may not have been Dash's, not to mention recalling a bad anecdote that Scoot and Dash both seem to have forgotten about...for some reason, it's a Troll Fic, it's best not to think about it. The point is Scoots reads the diary, is devastated at what she sees, and...well, just read for yourself:
"Scootaloo pushed the lever all the way forward. Rainbow Dash’s eyes grew to the size of pie plates as terror gripped her heart. She began to bang on the glass wall, hoping to break it, not realizing it was acrylic glass and therefore could not be broken by her. And the faster she banged on the glass, the faster she breathed in the foalmula gas. She began to cough at first and then felt weak at the knees. She fell back on her padded rear and began to cry. She cried like an infant pony, flailing her hooves in the air. The door to Scootaloo’s side opened and stepped out, leaving the wailing blue pegasus behind her."