candidate vs dark horse

candidate and dark horse both are nouns.

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

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

candidate: 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 candidate include favorite son, running mate, spoiler, stalking-horse, write-in, write-in candidate.

candidate (noun)	dark horse (noun)
someone who is considered for something (for an office or prize or honor etc.)	a racehorse about which little is known
a politician who is running for public office	a political candidate who is not well known but could win unexpectedly