You have multiple fields of vision, and one of them is called your peripheral vision. It begins at around 190 degrees. It’s far from perfect, sometimes leading us to see things that aren’t actually there. Other times it simply distorts what is. Here’s a great example: focus on the cross in the middle and watch the celebrity faces change.
Their faces start to look weird and distorted after a while, don’t they? It’s all because of an area of our retina called the yellow spot. It contains the most receptors in the eye, covering about 10% of our vision’s range. In the above situation, that main field is empty, leaving our range of vision relegated only to our less detailed peripheral vision.
The brain, having to process the images faster than it actually can, also distorts the images some. Because of this, we see the monsters!