bugbear vs monster

bugbear and monster both are nouns.

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

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

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

bugbear (noun)	monster (noun)
an object of dread or apprehension	(medicine) a grossly malformed and usually nonviable fetus
an imaginary monster used to frighten children	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