From this site: Do you remember what you were afraid of when you were a little child? What brought it on? How did you deal with it?
There are two that immediately spring to mind, and are linked. The first is dark--I've never been afraid of the dark, but I used to scare myself when there was dark behind me and light in front of me. One of the houses I lived in during my childhood had a downstairs bathroom. The stairway was an angled one; you know, four steps to a landing, then the rest of the flight at a right angle to the right. The light bulb was on the (second-story level) ceiling at the landing. So, coming upstairs from the bathroom (which was right at the foot of the stairs), there was a mass of blackness at my back, and a line on the landing whereupon it became bright. That never ever failed to freak me out; I always imagined how scary it would be if something was behind me in that dark. Not necessarily that I thought something was behind me, and not like I had a real imagination to create a darkness monster; the thought that an unknown and unseen creature could be there, and could then chase me, is what scared me.
The second thing I was "afraid of" was being chased. To some extent, I suppose I still am; no one really chases me anymore, though. My brother and I, or friends and I, used to always run around. There was always a point when I was being pursued, that the anxiety came to a head and the fun crossed into true fright. Of course I knew that there was no danger, it must have been instinctual. Usually I emitted some primal shriek, or at least wanted to. I don't know, it's hard to explain, but it almost always got to me if I was being chased long enough. Perhaps some of it was fear of actually being caught, but I also freak out about my footing. Like my scariest dreams have always been the ones where I can't run away or scream. So when I was being chased around the house, part of me was frightened that I would trip or suddenly not be able to move when needed. As I said, very possibly some kind of primal instinct.