My irrational fear is anything with a stinger.
I stay calm in the face of spiders, ants, cockroaches,maggots, whatever bug you can think of. But if a wasp or a bee flies by, I will be absolutely paralyzed and unable to breathe until the wasp or bee has gone away. Hives are unimaginably terrifying.
Tornadoes scare me to death as well. I can deal with hurricanes. I can deal with lightning and thunder. But I've had multiple nightmares about twisters chasing me across the plains in the midwest or something.
Alright, so these aren't exactly uncommon fears. But they manage to be the only things that can really and truly scare me so much that I'm incapitated.