dark horse; unexpected winner
Refers to a candidate or competitor who unexpectedly wins or succeeds, often used in elections, sports, and business contexts.
In this election, a dark horse candidate won.
He was a dark horse in the tournament, but he won.
Literally 'hole horse', from horse racing terminology where an unknown or overlooked horse (穴) unexpectedly wins. The exact origin of the metaphor is uncertain, but it is now widely used beyond racing.