lookdown fish vs horse-head

lookdown fish and horse-head both are nouns.

Word	Noun	Adjective	Verb	Adverb
lookdown fish	Yes	No	No	No
horse-head	Yes	No	No	No

As nouns, horse-head is a hypernym of lookdown fish; that is, horse-head is a word with a broader meaning than lookdown fish:

lookdown fish: similar to moonfish but with eyes high on the truncated forehead
horse-head: any of several silvery marine fishes with very flat bodies

Other hypernyms of lookdown fish include Atlantic moonfish, dollarfish, horsefish, horsehead, moonfish, Selene setapinnis.

lookdown fish (noun)	horse-head (noun)
similar to moonfish but with eyes high on the truncated forehead	any of several silvery marine fishes with very flat bodies