Some Diamond Dogs drank a small amount - not enough to cause transformation. When they died, they melted.
On that note, each nightmare the Mane cast has is made of this. It's not just the nightmare itself, but how broken they are afterwards. They're witnessing themselves betraying, torturing, and killing those closest to them, and they can't stop it.
The descriptions of the zomponies are made of Nightmare Fuel. Anyone affected suffers a Painful Transformation as their eyes dissolve, their flesh rots, and they scream. The pain is described as similar to being dipped in acid, and Medic can feel wind blowing through his ribcage.
So much of the Blank Story arc is Nightmare Fuel. When Grey Hoof has his Villainous Breakdown, his skeleton, and only his skeleton, grow to the size of an alicorn. He grows skeletal wings and a horn, too, and they all tear his flesh to shreds as they grow.
Then, mutant centipedes as big as a pony start crawling out of his back.
The song from "Melody of Agony – Part 1 and 2" To clarify: Anyone known for their loyalty, such as Rainbow Dash, suffers tremendous terror and pain just from hearing it. When it plays again, everyone affected suffers so much agony they're forced into a coma, the song strong enough to start to affect RED Sniper in the other world. And when it plays a third time, everyone affected falls under a suggestion that they have to kill whomever or whatever they were loyal to end the pain. This creates such delightful scenes such as Apple Bloom trying to kill her fellow Crusaders, Spike trying to kill Twilight Sparkle, BLU Sniper trying to kill his teammates, Mayor Mare trying to burn Ponyville down, Luna leading a second coup against Celestia, Mrs. Cake trying to murder her children while begging her husband to stop her...
"Tick tick, goes the clock..." Doctor Whooves' Painful Transformation into Nightmare Doctor. Minuette/Riversong, experiencing the the last few minutes of life of every version of herself that died a horrible, agonizing death, ones that she didn't regenerate from, at the same time, on eternal repeat. Demoman experiencing every time he died and forgetting each time he survived. Applejack having to live out Pinkie's and Twilight's Mind Rape dreams, both as victim and victimizer.
When Nightmare Moon possesses the Elements she's sent nightmares to, Pyroland starts rotting. In addition, Spy and Scout have mutated into disturbing monstrous forms.
Christian Brutal Sniper, the physical embodiment of Sniper's darkest fantasies and self-loathing.
Nightmare Gardener, made from poor sweet and kind Cheerilee.
Her transformation especially. Vine wings and a horn grow from her body, her blood turns green, and her personality is so utterly subsumed that her cutie mark alters.
Nightmare Gardener is made of a substance that is delicious to changelings. While there's no question she deserved being devoured, it's still a pretty awful way to go.
When BLU Engineer is attacked by a Weeping Angel, both Engineers are thrown far into the past, away from any hope of rescue. When we see them again, they've gone mad from the isolation and are working for Slendermane as Nightmare Clockwork and the Vagineer.
Essentially any time a TF2 Freakshow appears is going to be this... unless it's PAINisCupcake.
Nightmare Clockwork's first act upon directly engaging the heroes is to revert everyone who was a Nightmare at one point to that state. This carries over to their RED counterparts.
The Troubled Backstory Flashback to Barricade and the Doctor trying to apprehend Jeff the Killer. Barricade is Forced to Watch (the Doctor's blindfolded) as Jeff slowly tortures Barricade's seven - man squad to death.
When the protagonists visit Canterlot, they're dragged into personalized nightmares. Some of them are quite disturbing.
Princess Celestia is immobilized by thorny branches, surrounded by several reminders of the murder she may have accidentally committed.
The Master shows up to torment Derpy.
Soldier's confronted by the burning corpses of his old family.
Both Pinkie and Rarity face an Enemy Without from their nightmares.
Medic's assaulted by his past victims - over a hundred men and women.
Imagine having your home broken into by a complete stranger. Now imagine having said stranger try to rape your mother in front of you. Her rapist also happens to be a grown man armed with a knife. You have to resort to murder to protect your mother, despite being only a child. Scout's backstory is a whopping mixture of Nightmare Fuel, Tear Jerker, and Fridge Horror, and the worst part is that it could happen in real life.
The Difference Between Dreams and Reality takes place in a Cuckoo Nest that's trapped all of Ponyville, save for those not native to the universe. It's scarily plausible.
Princess Celestia is a gibbering mess in the Cuckoo Nest. Seeing how reliant on Slendermane's power the Big Good has become is terrifying. What's even worse is that she seems almost fine in reality.