I'm afraid of cockroaches, and anything vaguely cockroachy, like water bugs. Some other things depend on context. For example, if I see a large spider in my room, I'll be scared and imagine finding a nest of them or something, but if I just see one outside I don't care. Any kind of large creature becomes more scary when it's where I live (I only really mind cockroaches and wasps otherwise). Potato bugs, actually, are the worst of them all in my opinion, but they don't live where I do, so yay! I just saw them when I was visiting my aunt in California.
I'm also scared of heights, and I thought it had no origin, but apparently it developed when I fell down some stairs when I was really little.