flathead vs percoid fish

flathead and percoid fish both are nouns.

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

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

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

Other hypernyms of flathead include percoid, percoidean.

flathead (noun)	percoid fish (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