Bees. When I was two or three years old, I was playing in the driveway and I got stung on the throat by a bee. It started to swell up so bad that I began suffocating. The obvious conclusion is that I was deathly allergic to them. I've been stung a few times after that it's never been that serious, but still knowing that I almost died at an absurdly young age because of that kinda haunts me whenever I see them buzzing about their day.

Also cockroaches because they freak me the hell out, though at least I moved to a part of the US that they don't inhabit.