Reminds me of a ridiculous fear I had as a little kid. I had imagined a creature called "The Dark" (essentially a shadowy horror that resembled a rotting humanoid raven dressed in black druid robes) lived in the deepest, darkest woods, but also spied on little kids from the edge of any wooded area.
I had nightmares for months that it was trying to bite me through my bedroom window at night.

This inspired me to design a horrific fallen angel for 2E AD&D Planescape. It killed of the entire party.

What inspirations have you had for spooky monsters?