butterfly fish vs percoid

butterfly fish and percoid both are nouns.

Word	Noun	Adjective	Verb	Adverb
butterfly fish	Yes	No	No	No
percoid	Yes	No	No	No

As nouns, percoid is a hypernym of butterfly fish; that is, percoid is a word with a broader meaning than butterfly fish:

butterfly fish: small usually brilliantly colored tropical marine fishes having narrow deep bodies with large broad fins; found worldwide
percoid: any of numerous spiny-finned fishes of the order Perciformes

Other hypernyms of butterfly fish include percoid fish, percoidean.

butterfly fish (noun)	percoid (noun)
small usually brilliantly colored tropical marine fishes having narrow deep bodies with large broad fins; found worldwide	any of numerous spiny-finned fishes of the order Perciformes