Lately, I've had a real fascinating with monsters and other sorts of cosmic horrors. It's probably because of the Lovecraftian-esque genre of movies, books and games I've seen recently and in the past. Amnesia: The Dark Descent, Silent Hill and The Colour out of Space just to name a few. I guess any sort of unexplainable or enigmatic beings we can't describe or even comprehend frightens us. We can explain angels, demons and spirits (for the most part) because they're almost human or have human-like characteristics or behaviors. But monsters? How can we put it into something we can understand? How can we describe a monstrosity like Cthulu?
Re: Monsters and the human psyche Cockroach isn't a monster EDIT: At least when specifically talking about the actual genre of monsters. Maybe in your own mind, that's what your mind discerns as a fear or threat.
I always thought of outlandish monsters like that as putting abstract fears we can't really process into something observable.