boogeyman vs monster

boogeyman and monster both are nouns.

Word	Noun	Adjective	Verb	Adverb
boogeyman	Yes	No	No	No
monster	Yes	No	No	No

As nouns, monster is a hypernym of boogeyman; that is, monster is a word with a broader meaning than boogeyman:

boogeyman: an imaginary monster used to frighten children
monster: an imaginary creature usually having various human and animal parts

boogeyman (noun)	monster (noun)
an imaginary monster used to frighten children	(medicine) a grossly malformed and usually nonviable fetus
	a cruel wicked and inhuman person
	a person or animal that is markedly unusual or deformed
	an imaginary creature usually having various human and animal parts
	someone that is abnormally large and powerful