The scariest monsters are the ones you never see. They haunt the shadows of your mind, and creep in the unlit corners of your darkened room. You never see them, but you think can hear them. You can feel them just beyond your senses, waiting and watching. They are the fear that crawls up from deep inside you as you find yourself unable to run. If you run, then they will know that you know they are there. So you walk slowly with stiff shoulders and white knuckled fists, hoping and praying it is not time yet for them to take you.
So turn off all the lights in your house, and take a step out into the darkness outside. Now turn your back to that dark and close your eyes as you hold your breath. If you are quiet enough, you will hear it coming. You will feel it reach for you as the thundering in your heart drowns out its final approach. The tiny hairs on the back of your neck raise as the unseen terror pauses in anticipation an inch from your skin...
Now go back inside, but never turn around. It was not your time, but do not temp them by showing your eyes. Never turn around. They can see fear.
In closing, the most terrifying monsters are those we create in our own minds.