Mine is a common one: spiders!
Yeah, I know...very few are harmful but I've been told that all have some degree of venom. Most won't bite unless 'cornered', and if I understand correctly, the arachnid (not a true spider) called a "Daddy Longlegs" (common in the south) has such tiny fangs it cannot penetrate human skin. I also read that these creatures are the most venomous arachnid known to exist, and they eat spiders, especially the more toxic types(Black Widow; Brown Recluse; etc...)
I think my phobia started due to being bitten by a Brown Recluse when I was 9. I removed a drawer from a very old dresser and there was a card in the back. I reached in to retrieve it and felt what I thought was a splinter. Within a few hours, my hand had swollen to several times its normal size; one fingertip was black and disgusting, and the others looked discolored, too. It was during the night, so I woke up my mom and told her my hand hurt. She was half-asleep, never opened her eyes and said: "run some cold water over it."
By morning, my fingernail had fallen off of that finger, the tip of my finger was badly necrosed (rotting) and the rest of the nails on that hand had turned black. I eventually lost all of the nails on that hand, and my finger and nail are still very slightly deformed.
A few months later, we had just moved into a new home. I walked onto the front porch then ran back in and told my mom there was a black widow by the door. She rolled her eyes at me with that, "yeah, sure" and followed me out. Showed her! It was an average sized female black widow who was in the process of dining on her mate. That is a vision which still haunts me.
To this day, I freak out at the sight of any spider...the bigger the spider, the worse I freak out. I usually have someone else dispose of it, even if I have to go ask the little girl next door...I can't do it. A few months ago, there was a harmless common wolf spider on my counter. I had no one to rescue me, so I HAD to kill it myself.
I squished it (it managed to get a couple of 'hops' in before I finally was able to do so) and it was so full of goo that it splattered. I immediately threw-up. I couldn't clean it up, so I put a dozen or so paper towels on it then stayed out of the kitchen entirely until I could find someone else to do it.
Yes, my brain knows that the chances of being bitten by anything that would cause more than a brief sting and almost nil...but my instinct says "death on 8 legs". Strange thing about it, it seems to be getting worse as I get older.