The episode "Johnny Test: Party Monster", in which Susan and Mary test a substance named Phero-Booster on some mice and Johnny to make them look good. It works at first, but when they want to use it themselves, they discover that both the mice and Johnny turned into hideous, disfigured monsters. When the party guests were on the verge of leaving the party, the sisters' antidote is an invention that transfers their good looks to Johnny, causing them to become the same kind of monsters. At the end, when Dukey calls them boogers, they pull a lever that opens a door underneath him, making him fall.
The episode "Johnny's Nightmare House" has Johnny inadvertently turning the house into a haunted one for mistaking Hugh's nightmare diary for a book which contains his father's plans for a dream house. Some of the terrifying monsters from Hugh's nightmares include a weird-looking, purple-furred, three-headed creature and a giant spider with sharp teeth and three red eyes.
The episode "Johnnyitis", where Johnny drinks an unstable isotope and almost blows up.
Johnny becoming a hulk-like beast after getting much testosterone in him in "Johnny Testosterone".
Dukey: Okay, I'm officially scared.
"Johnny's Royal Flush" has a disturbing scene where an alligator gets sucked into a sewer turbine and is promptly ripped to shreds, which is followed up by revealing that it got turned into alligator-skin bags.
Poor Gil being very terrified when the now vampire Susan and Mary try to bite him and have him for the rest of eternity in "Fangs a Lot Johnny".
