noun
a candidate or competitor about whom little is known but who unexpectedly wins or succeeds
adjective
relating to a little-known competitor who unexpectedly wins or succeeds
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The dark horse in the election surprised everyone with a landslide victory.
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I never expected him to win, he was a real dark horse in the competition.
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That team is definitely the dark horse in the tournament, they could go all the way.
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She always seemed like a dark horse in the family, but she ended up being the most successful.