underdog vs nonstarter

underdog and nonstarter both are nouns.

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

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

underdog: one at a disadvantage and expected to lose
nonstarter: a person with a record of failing; someone who loses consistently

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

underdog (noun)	nonstarter (noun)
one at a disadvantage and expected to lose	a horse that fails to run in a race for which it has been entered
	a person with a record of failing; someone who loses consistently