Spiders.
It is thought that the widespread, often irrational fear of spiders is an evolutionary remnant behaviour.
I could do without it though. A tiny spider, does nothing. But even a quite small bigger spider, the ones where they are big enough for you to see the eyes and hair on the legs, they instill a sort of primordial revulsion and fear in me. I can not be in the same room with one until it is dead. A feat usually achieved by throwing a large book at it from quite a way away.
It is a completely irrational fear, where I live it is too cold for dangerous spiders to occur and the biggest ones we get are cave spiders, the ones that moved from the caves into houses with us.
Mind, they can live to be thirty and never stop growing so they can end up bigger than your hand. The old ones almost never show themselves to people though, they got old cause they're wily.
But even these are of no danger to people. Still, hairs stand on end not just on the back of my neck but over my entire body and I am gripped with a panic, it is undescribable.
All the while knowing they are harmless.
Because of my fear of spiders I have become quite fascinated with them and have studied up on them every opportunity.
There are some really amazing spiders out there in the world, spiders that use a strand of web to lassoo prey with, spiders that build their nest under water, solitary hunting spiders with a brain the size of a grain of sand who exhibit complex hunting behaviour you'd normally expect from a tiger or wolf.
Fascinating creatures really, I just don't want to see them in my house. Or in a zoo or pet store. I get the heebee jeebies even from television and pictures in books, but I dont let that stop me.
If you give in to fears like this you are re-enforcing neural pathways and in effect making your phobia worse. The worst thing anyone with any phobia can do is to totally and completely avoid all triggers of said phobia. (Which is why I was always really opposed to those 'no spiders please' requests before Skyrim was released.)