merman vs imaginary being

merman and imaginary being both are nouns.

Word	Noun	Adjective	Verb	Adverb
merman	Yes	No	No	No
imaginary being	Yes	No	No	No

As nouns, imaginary being is a hypernym of merman; that is, imaginary being is a word with a broader meaning than merman:

merman: half man and half fish; lives in the sea
imaginary being: a creature of the imagination; a person that exists only in legends or myths or fiction

Other hypernyms of merman include imaginary creature.

merman (noun)	imaginary being (noun)
half man and half fish; lives in the sea	a creature of the imagination; a person that exists only in legends or myths or fiction