Well, I came into this thread thinking of lighthearted stuff and people got all deep on me.
Lighthearted fears:
Spiders and bugs: Mostly from my parents basement and cockroaches and spiders and stuff. The middle of Temple of Doom didn't help either.
Creepy Clowns: Not regular clowns who look nice and happy, but IT and John Wayne Gacy and really freaking creepy clowns.
Deep:
Definitely agree with the fears involving your kids and family. Sucks.
One specific fear started a few years ago and has to do with my kids.
My older boys were 4 and 6 when their mom and I decided to split for good. That really tore me up. I stayed with her for a lot longer than I should have because of my kids. The day I moved out of the house was one of the worst days of my life. It didn't help that my ex said she would keep the kids away until I was gone, but she decided I'd had enough time and came home anyways. Thanks for that.
My fear stems from that split. I am horrified that our divorce has caused emotional trauma to my boys. Middle boy was only 4 at the time, so I don't think he understood it that well, but he's still a pretty immature kid. He's 9 now, so he's got time to adjust. Oldest was 6 so he was there for a lot of the bad days leading up to the split. He was having emotional outbursts then and I'm afraid it was in reaction to the tension between his mom and I. He's still a good kid, but he seems like he has some empathy issues. That might just be part of his personality or maybe his mothers influence, but he's a little less care free than I wish he could be.
So there's my debby downer, hopefully irrational fear.