bullfrog vs true frog

bullfrog and true frog both are nouns.

Word	Noun	Adjective	Verb	Adverb
bullfrog	Yes	No	No	No
true frog	Yes	No	No	No

As nouns, true frog is a hypernym of bullfrog; that is, true frog is a word with a broader meaning than bullfrog:

bullfrog: largest North American frog; highly aquatic with a deep-pitched voice
true frog: insectivorous usually semiaquatic web-footed amphibian with smooth moist skin and long hind legs

Other hypernyms of bullfrog include ranid.

bullfrog (noun)	true frog (noun)
largest North American frog; highly aquatic with a deep-pitched voice	insectivorous usually semiaquatic web-footed amphibian with smooth moist skin and long hind legs