flathead vs percoid

flathead and percoid both are nouns.

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

As nouns, percoid is a hypernym of flathead; that is, percoid 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: any of numerous spiny-finned fishes of the order Perciformes

Other hypernyms of flathead include percoid fish, percoidean.

flathead (noun)	percoid (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