Dark horse
Overview
 
Dark horse is a term used to describe a little-known person or thing that emerges to prominence, especially in a competition of some sort.
The term began as horse racing parlance. A dark horse is a race horse that is not known to gamblers and thus is difficult to place betting odds on.

The earliest-known use of the phrase is in Benjamin Disraeli's novel The Young Duke (1831).
 
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