flathead vs percoidean

flathead and percoidean both are nouns.

Word	Noun	Adjective	Verb	Adverb
flathead	Yes	No	No	No
percoidean	Yes	No	No	No

As nouns, percoidean is a hypernym of flathead; that is, percoidean is a word with a broader meaning than flathead:

flathead: pallid bottom-dwelling flat-headed fish with large eyes and a duck-like snout
percoidean: any of numerous spiny-finned fishes of the order Perciformes

Other hypernyms of flathead include percoid, percoid fish.

flathead (noun)	percoidean (noun)
food fish of the Indonesian region of the Pacific; resembles gurnards	any of numerous spiny-finned fishes of the order Perciformes
pallid bottom-dwelling flat-headed fish with large eyes and a duck-like snout