I accept this is very niche, but those freaky radiation-mutants in 'Beyond the Planet of the Apes'.
I watched it when I was about 8 and the bit when they take their 'human' masks off left me messed up. I thought they were the good guys who could kill all the evil apes (oh, how simplistic we are when we're kids) then they go and pull that shit!
I also had a bit of a problem with Daleks. Daft part is, I wasn't scared of them that much when watching them on the telly. I'd have occasional nightmares about them that left me unsettled for days (there's one I still remember about 40 years on - I hid in the drawer in my bed but they found me and killed me - but all that happened was I stood up and followed them round chatting with them as they went about their extermination business, so I wasn't actually dead, just sort of out of the game of survival). Worse, I used to get a bit angsty about big cylindrical things - like those great big commercial dustbin things that were on wheels or pillars in general. We're talking when I was about 5 or 6. More than once I started to cry because of them.
I have a nagging feeling there's something else buried in my memory, but it's being well repressed.