Clowns were mine - frightful scary beings that act superfriendly? Nearly killed my introverted, shy childhood. (I have no problem with them now - I think since one of my montessori teachers was one and had showed us how she put her make up on and stuff.)
Slasher/gore movies, as well as my mother's anatomy books. Apparently, my teachers were concerned (alarmed) that I did not share my classmates' curiosity on how the body looked and worked on the inside and would violently 'put up a wall' when we studied it. I recall having frequent nightmares about innards all over the place. (Still haven't gotten over it - STILL sends me into a panic attack with just glimpsing anything.)
Earthquakes. I lived in Missouri all of my childhood, so there shouldn't be a reason, but the lates 70's/early 80's had those rash of disaster movies ... (I live in FL, so I fear hurricanes now.)
I could go on, as I'm mildly neurotic, but those were the big three.