Back when I was very young, like still in elementary school, I started playing Super Metroid for the first time. I was running around Norfair, generally having a good time, when I opened an innocuous-looking super missile door and encountered... Crocomire.
If you're familiar with that game, you might be thinking "Oh, you got scared when Crocomire's skeleton popped out of the lava, right?" No, I never even made it that far. Instead, on finding myself suddenly confronted by this huge, grotesque monster to a background of intense, blaring music, I freaked out. I didn't even think of aiming for his mouth, instead just firing all my missile and super missiles in a blind panic as he slowly pushed me toward the spiked wall at the far end of the room. At the time, I didn't know you could hurt bosses with the charge beam, so once I was out of missiles, I used the only other weapon I thought might work: a power bomb. Turns out that's the worst possible thing you can do. Power bombs make Crocomire go apeshit, charging at you and quickly smashing you into the rear wall, at which point you're basically guaranteed to die. Sure enough, I did.
I didn't quit the game forever, but I did put it give up on progressing further until I was older, wiser, and generally better at video games. And even then, for years afterward, I played the Crocomire fight with the volume off.
Courage the cowardly dog scared the shit out of me, but specifically there was an episode where anthropomorphic eggplant try to take over the world. I couldn’t eat eggplant for years after that, and I still mostly avoid it
Return the slab was my one. Would scare me to tears lol.