A Dark Horse

 
To refer to something or someone as a dark horse is to suggest they have come to prominence from relative obscurity. The phrase was originally used in relation to a real horse. It was not uncommon for horse owners to hide the potential of their best horses by keeping them hidden until the day of the race. A 'dark horse' was therefore one that wasn't known to the punters and was difficult to place odds on. The figurative use later spread to other fields and has come to apply to anyone who comes under scrutiny but is previously little known.

Benjamin Disraeli's novel The Young Duke, 1831, provides the earliest known example : "A dark horse, which had never been thought of ... rushed past the grand stand in sweeping triumph."