unicorn vs imaginary creature

unicorn and imaginary creature both are nouns.

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

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

unicorn: an imaginary creature represented as a white horse with a long horn growing from its forehead
imaginary creature: a creature of the imagination; a person that exists only in legends or myths or fiction

Other hypernyms of unicorn include imaginary being.

unicorn (noun)	imaginary creature (noun)
an imaginary creature represented as a white horse with a long horn growing from its forehead	a creature of the imagination; a person that exists only in legends or myths or fiction