merman vs imaginary creature

merman and imaginary creature both are nouns.

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

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

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

Other hypernyms of merman include imaginary being.

merman (noun)	imaginary creature (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