Dark horse

From Fresh Dictionary

English

Etymology

Originally an allusion to an unknown horse winning a race, as used in an 1831 novel The Young Duke by Benjamin Disraeli.

Noun

dark horse

1. ((idiomatic{{#if:|, {{{2}}}{{#if:|, {{{3}}}{{#if:|, {{{4}}}{{#if:|, {{{5}}}{{#if:|, {{{6}}}{{#if:|, {{{7}}}{{#if:|, {{{8}}}{{#if:|, {{{9}}}}}}}}}}}}}}}}}}})) (politics): A candidate who is nominated unexpectedly, without previously having been discussed or considered as a likely choice.
2. (rare): An unexpected success

   Everyone was expecting the red team to win, but the greens were the dark horse in the event.