monster under the bed

monster under the bed (plural monsters under the bed)

1. Used other than figuratively or idiomatically: see monster,‎ under,‎ the,‎ bed.
   • 2007, Jason Edwards, Will Allen and the Great Monster Detective‎, page 45:

     You've been afraid of broken toilets since you were three. How long have you had a monster under your bed?

2. An unreasonable and childish fear.

This is not a fixed expression; one can also say monster under my bed as appropriate, rather than something such as  my monster under the bed.