A dark horse

Also found in: Idioms.
References in classic literature ?
The third, a bit of a dark horse from the first, was at the end of eighteen months something of a dark horse still to the department.
Any digital comic entitlement added to a Dark Horse Digital account during April 25, 2014, to April 27, 2014, will constitute an entry to win.
com)-- Helen Darras, Licensor & Producer for a Dark Horse Comics Graphic Novel will be appearing as a guest in Wales at the Inaugural 2013 Cardiff Film & Comic Con, held August 31 - September 1.