Many people have legitimate phobias, which by definition are irrational and do not have a root cause. Likely it's something in the psyche that evolved as a result of the fact that there are species that can be fatal to humans.

When my sister had a snake in her classroom her students told her they found it creepy because you can't see how they move. They just seem to...go. I can see that. Plus they bite. And it's rather violent and startling when they eat something live.