bug-a-boo

noun2

1. •
   A mythical, nocturnal creature; a hobgoblin.

   • The German ubu, as well as the French bibou, is also used for bug-a-boo, hobgoblin, or any other fantastical, terrific nocturnal object. — 1776
2. •
   Any imagined fear or threat, or a fear presumed larger than it really is.

   • ...a fear had come upon them, and they had a kind of bug-a-boo terror about roving gangsters. — 1949