dark horse

WordReference Random House Learner's Dictionary of American English © 2025

ˈdark ˌhorse, n. [countable]

1. Sport a racehorse, competitor, etc., that is somewhat unknown or that wins unexpectedly.

WordReference Random House Unabridged Dictionary of American English © 2025

dark′ horse′, 

1. Sport a racehorse, competitor, etc., about whom little is known or who unexpectedly wins.
2. Government a candidate who is unexpectedly nominated at a political convention.
• 1825–35

Collins Concise English Dictionary © HarperCollins Publishers::

dark horse noun

1. a competitor in a race or contest about whom little is known; an unknown
2. a person who reveals little about himself or herself or his or her activities, esp one who has unexpected talents or abilities
3. a candidate who is unexpectedly nominated or elected