What is the meaning of the idiom "a dark horse"?

The answer is ☀ An unforeseen competitor.

The idiom "dark horse" refers to a competitor who is not well-known or expected to win, but who unexpectedly succeeds. The phrase originated in horse racing, where a dark horse is a horse that is not well-known or has not been racing often. However, the idiom is now used more generally to refer to any unexpected winner or competitor.

The other options are incorrect. A formidable competitor is someone who is very strong or skilled, and a nightmare is a frightening or unpleasant experience. A handsome fellow is a good-looking man.