Plucky channeling Einstein in "Never Too Late to Loon" is a direct shout out to the Pink Elephants on Parade scene from Dumbo. It's equally mindscarring.
Wackyland and its surreal humor. They try to have Babs decapitated for having self control.
The earthquake in "Journey to the Center of Acme Acres," as well as the Gremlins.
One segment of the Halloween episode "Toons from the Crypt" had Elmyra, an animal lover whose affection was always disastrous for the target animal, mourning her late pets... whereupon they rose from the dead Zombie Apocalypse-style and came after her. Their being zombified didn't prevent her from lavishing her destructive affections on them, however. One truly scarring moment shows her plunking a zombie dog and cat into a tub of soapy water to give them a bath. But then she pulls out her hands and they're covered in muck. In other words, the zombie pets fell apart in the water.
The first segment of the episode, titled "Wait 'Till Your Father Gets Even", is pretty scary too, especially the ending, where Plucky is tied to a board being cut in half by a saw as punishment for gambling.
In "Fit to Be Stewed," a witch named Sandy tried to cook Buster and Babs in a stew—until she realized Babs was a cartoon rabbit and her recipe called for a real rabbit. So she used her magic to turn Babs into a helpless real rabbit, unable to save herself.
A variation on the usual Michigan J. Frog shorts. In this one, the frog was dead and awaiting dissection in a biology class, but came back to life when the teacher's back was turned. It was immeasurably creepy – especially the ending, where Hamton tried to bury it in a pet cemetery, but all the other animals came back to life... and sing.
Wolvertooth from the Pluck Twacy short, which basically consisted of a closeup of Bugs Bunny grinning with a bunch of sharp pointed teeth.
"Slaughterhouse Jive" as a whole can get pretty freaky. This is one of those things that can scare most meat-loving children into becoming vegetarians and at the same time scare kids away from vegetables.
"Sawdust and Toonsil" is easily one of the darkest episodes Tiny Toons has ever produced. The story starts with Silas Wonder's circus coming into town, and the man himself invites Buster, Babs, and Plucky in to see his "wonderful wonderpieces". Then everything goes downhill. By the end of the episode, Silas Wonder is revealed to horribly, HORRIBLY mistreat the animals he has in his circus (and it's practically said that he poached them) and gets Killed Off for Real, the ONLY Tiny Toons villain to actually die.
"One Beer" was a parody of the Very Special Episode that demonizes alcohol and ends with everyone dying (even though Buster, Plucky, and Hamton reveal that the whole thing was just part of the show, it did get complaints for cheapening the Aesop that "Underage drinking isn't cool and will lead to a life of ruin" and was banned until the 2000s and 2010s, when it was found on the DVD release of the show and in reruns on The Hub).
In "The Undersea World of Fifi," Elmyra is desperate to catch sea monkeys. She ends up finding a plug at the bottom of the ocean, which pulls everything, even the universe, into it. "But at least I got my monkey-wonkey!"
The same goes for the ending to the episode with Duck Dodgers in it, and that ending makes it even worse.
Mr. Hitcher from How I Spent My Summer Vacation qualifies as he's a parody of Jason Voorhees and John Ryder. The whole extended sequence where he chases Buster and Babs even has him swinging his chainsaw wildly about, which causes part of the mine to collapse. And he does it with a really deranged look on his face.
The episode "Hare Raising Night" features Dr. Gene Splicer, who in his very first scene is described as a certified loony who performs deranged animal mutation experiments. Cut to three guinea pigs, who, when Splicer opens the cage, are seen cowering in fear as he takes one guinea pig away (to be presumably experimented upon) as the camera cuts to static. It only gets worse when Buster and group arrive to stop him (actually just Buster; he tricked Babs, Plucky, and Hamton into coming), as he takes Plucky away for his brain, is extremely abusive to his creation Melvin, and at the end tries to mutate the four of them, and then ends up falling into his own vat of mutation goo. However, his Crowning Moment Of Nightmare comes in what he says after trapping the four in his Animal Experiments Lab and talking to them via a creepy-as-hell distorted static-y image of his face (which is then zoomed in upon for good measure):
Dr. Splicer: Aah, guess again kiddies. I got a little riddle for ya: what do you get when you cross two bunnies, a duck, and a pig?
Buster: I don't know.
Dr. Splicer: Neither do I, but we're gonna find out!
The segment "Duck In The Dark" from "Best O' Plucky Duck Day", in which Plucky keeps having nightmares after watching a marathon of horror movies, the scariest bits are at the beginning in the movie Plucky watches as a fawn is stalked and presumably killed offscreen by Eddie Couger (A Freddy Krueger parody), and later when Plucky tries to escape from Eddie and tries to wake up Buster only to discover that he's turned into a horrifying monster with an eyeball hanging out of one his sockets. And at the very end after the nightmares are over there's the angry Nightmare Face Buster makes after learning Plucky learned absolutely nothing from them and has gone back to happily watching the horror films.
The entire "Boo Ha Ha" segment from "Return of the Acme Acres Zone" where a ghost terrorizes Plucky and Hamton. The different Eldritch Abomination forms he takes throughout the short are horrifying. And then there's the ending, where he takes them to hell with them. The End!
The effing Banshee in the second "Viewer Mail Day" segment.
The "find the object" game Buster and the Beanstalk isn't too frightening for the most part, but as the manual warns you, Elmyra is hunting down Buster and Plucky as they search for key segments. Once she's closing in, she'll taunt you from parts unknown; you'd better find what Babs tasked you with retrieving before Elmyra materializes out of nowhere to capture the boys and lock them up in an endless kennel.