white whale vs dolphin

white whale and dolphin both are nouns.

Word	Noun	Adjective	Verb	Adverb
white whale	Yes	No	No	No
dolphin	Yes	No	No	No

As nouns, dolphin is a hypernym of white whale; that is, dolphin is a word with a broader meaning than white whale:

white whale: small northern whale that is white when adult
dolphin: any of various small toothed whales with a beaklike snout; larger than porpoises

white whale (noun)	dolphin (noun)
small northern whale that is white when adult	any of various small toothed whales with a beaklike snout; larger than porpoises
	large slender food and game fish widely distributed in warm seas (especially around Hawaii)