A dark horse is an unexpectedly strong candidate or contender for victory, whose claim to victory has not been discussed before. The term comes from equestrian sports, where a dark horse was called an unexpected winner in a horse race. The most previously known mention is in Benjamin Disraeli's novel "The Young Duke": "A dark horse, which no one could have thought of, and which the careless Duke of St. James did not even notice in the list, triumphantly raced past the stands"
And a piano in the woods. Ryo - a dark horse that appeared only in the last chapter