Dark horse
Etymology
From horse racing, where a "dark" horse was one whose form and breeding were unknown to bettors, making its odds a guess. Benjamin Disraeli popularized the phrase in his 1831 novel "The Young Duke," describing a race won by an unconsidered outsider. It now names any surprise candidate in elections, hiring, or awards.
noun phrase · /DARK-hors/
A little-known contender whose abilities are unknown and who may surprise everyone by winning.
What it names
The risk and promise of the unassessed competitor — the one nobody has priced in.
Example
A dark-horse candidate from a tiny district suddenly led the primary field.
Related idea
A dark horse is unknown, so anything is possible, while an underdog is known to be weaker and expected to lose.