There's a Monster in My Closet (thing)

A children’s book by Mercer Mayer, written in 1968. It is about a young boy who knows there is a monster in his closet, so he waits up late and shoots him with a cork-gun. The monster cries and the boy ends up comforting him, and they become friends.

I remember my younger brother was so scared of the illustration of the monster that he hid this book under his bed for weeks.