I was a really easy to scare child.
I remember being like 6 or 7 and being really afraid to swim to the far end of the pool alone while taking swimming lessons, because this Yu-Gi-Oh card would always pop into my head.
Also, does anyone else remembers this final boss from a shmup called Strikers 1945 2? I recently discovered it was not some weird nightmare I had as a kid.
ReDeads in Ocarina of Time absolutely fucked me up as a kid. Horrifying.
I remember when I first pulled the Master Sword from its pedestal and thought "Hell yeah, time to kick some ass" and then I went out and the town was destroyed and full of those things. I was scared to play the game after that.