Creep

Creeps are creatures that prey on, harass or otherwise scare young children. They are first described in the short story "Zoo Day".

Description
Creeps can be clearly seen only by children; adults almost always fail to perceive them. Even Mouse has difficulty to notice them. It is possible there is a number of varieties, but only one is described in some detail.

"Zoo Day"
In "Zoo Day", Margaret Angelica Dresden is attacked by a group of them, whom she calls haunts. She manages to defeat them by facing up to her fears and conquering it.