If you describe someone as a dark horse, you mean that people know very little about them, although they may have recently had success or may be about to have success.
dark horse in British English
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. US politics
a candidate who is unexpectedly nominated or elected
dark horse in American English
1.
an unexpected winner in a horse race, thought beforehand to have very little chance
2.
an almost unknown contestant regarded by few as a likely winner
3. US, Politics
a person who gets or may get the nomination unexpectedly, often by a compromise
Examples of 'dark horse' in a sentence
dark horse
Rice is the dark horse of the race.
Times, Sunday Times (2011)
The dark horse of the race is a bright Vatican diplomat.
Times, Sunday Times (2009)
But Bird has been mentioned as a dark horse in the race.
Times, Sunday Times (2014)
The midfielder already has senior caps and his free-scoring side could be one of the dark horses.
The Sun (2015)
Talk about a dark horse.
Times, Sunday Times (2012)
I would say this lot could go either way - they may be a dark horse.
The Sun (2010)
Outside Britain also it has been a case of white horse good, dark horse problematic.
Times, Sunday Times (2008)
France's preparations have been anything but smooth and they will face a Uruguay team who are being tipped as one of the dark horses.