nominee vs dark horse

nominee and dark horse both are nouns.

Word	Noun	Adjective	Verb	Adverb
nominee	Yes	No	No	No
dark horse	Yes	No	No	No

As nouns, dark horse is a hyponym of nominee; that is, dark horse is a word with a more specific, narrower meaning than nominee:

nominee: a politician who is running for public office
dark horse: a political candidate who is not well known but could win unexpectedly

Other hyponyms of nominee include favorite son, running mate, spoiler, stalking-horse, write-in, write-in candidate.

nominee (noun)	dark horse (noun)
a politician who is running for public office	a racehorse about which little is known
	a political candidate who is not well known but could win unexpectedly