dark horse

noun3

1. •
   Someone who possesses talents or favorable characteristics that are not known or expected by others.

   • ‘She’s a dark horse,’ he said. ‘She knows just as much about climbing mountains as you or I. In fact, she was ahead of me the whole time, and I lost her.’ — 1952
2. •
   A candidate for an election who is nominated unexpectedly, without previously having been discussed or considered as a likely choice.

   • That left Cardinal Robert Francis Prevost, 69 — a quiet American dark horse who had, surprisingly, emerged in the evening’s vote — as a source of particular interest. — 2025
3. •
   A horse whose capabilities are not known.