There is a lot that frightens Stephen King. His most common response when asked why he writes in the horror genre is What makes you think I have a choice?, and many of his stories have been inspired by his own fears. He has a fear of therapists, which he had to overcome when he was battling his drug and alcohol addiction back in the day.
One day he hopes to write a horror story centred solely on spiders, since those are the things that scare him most whilst most people think the ending to It is a bit of a damp squib, to King it's probably the most terrifying thing imaginable. He's had a fear of choking since his mother died on the night his kid had a choking fit.
But most extreme of all his not-so-irrational fears is his aversion to flying. He already didn't do well with planes; then, after begrudgingly getting on one flight, it hit turbulence. The oxygen masks were deployed. And it got so bad that his seat was torn from the floor and tipped over with him still strapped in. Took him a while to get on a plane again.