underdog vs failure

underdog and failure both are nouns.

Word	Noun	Adjective	Verb	Adverb
underdog	Yes	No	No	No
failure	Yes	No	No	No

As nouns, failure is a hypernym of underdog; that is, failure is a word with a broader meaning than underdog:

Other hypernyms of underdog include loser, nonstarter, unsuccessful person.

underdog (noun)	failure (noun)
one at a disadvantage and expected to lose	an act that fails
	an unexpected omission
	an event that does not accomplish its intended purpose
	a person with a record of failing; someone who loses consistently
	lack of success
	loss of ability to function normally
	inability to discharge all your debts as they come due