Dark Horse Meaning

Example : 2005, Steve Augarde, Celandine, London: Corgie Books edition (2006), 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?"2009, Sophie Kinsella, Twenties Girl, London: Black Swan edition (2010), ISBN 978 0 5527 7437 6, page 183:  "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!"