dark horse

musta hevonen puhekieltä Someone who possesses talents or favorable characteristics that are not known or expected by others.
2005, (w), w:Celandine (novel)|Celandine, London: (w), Special:BookSources/9780385609180 ISBN 978-0-385-60918-0; republished London: w:Transworld Publishers|Corgi Books, 2006, Special:BookSources/9780552549684 ISBN 978-0-552-54968-4, page 13:

As she pulled the door closed behind her, she heard the nurse say, “Well! You’re a dark horse, I must say! Do you know that extraordinary-looking girl?”

“Well!” Genevieve laughs – the kind of bright, trilling laugh you give when you’re really quite annoyed about something. “Ed, you are a dark horse! I had no idea you had a girlfriend!”