dark horse (n.)youdaoicibaDictYouDict[dark horse 词源字典]
in politics, 1842, an image from horse racing, in which dark is used in its figurative sense of "unknown."
Moonraker is called a "dark horse"; that is neither his sire nor dam is known. ["Pierce Egan's Book of Sports," London, 1832]
[dark horse etymology, dark horse origin, 英语词源]