nonstarter vs underdog

nonstarter and underdog both are nouns.

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

As nouns, underdog is a hyponym of nonstarter; that is, underdog is a word with a more specific, narrower meaning than nonstarter:

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

Other hyponyms of nonstarter include bankrupt, insolvent, flash in the pan, dud, flop, washout.

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