Anything can be looming in the darkness.  It's that uncertainty which instills fear in so many people.  One of the most common forms of this fear is achluophobia or the fear of the dark. 



Horror movies often utilize the prevalence of this fear among people to effectively terrify them.  When the main characters are running through the forest, it is even scarier because anything can happen. This fear is considered rational because the idea that anything could happen could bring harm to someone, therefore making the fear logical.