(A). A racing term for a horse of good pretensions, but of which nothing is positively known by the general public. Its merits are kept dark from betters and book-makers.
“At last a Liberal candidate has entered the field at Croydon. The Conservatives have kept their candidate back, as a dark horse.” —Newspaper paragraph, January, 1886.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894