The history of the word derby is intertwined with the history of the horse races that are held annually.
In the early 17th century, shoo began to be used as a transitive verb, as in "He shooed the pigeons away from the bench." Later, the verb developed an intransitive use reflecting the result of shooing, as in "When she hollers at the cat, it shoos at once." It is around 1900 that the verb began to be used figuratively in horse racing.
Originally, the term referred specifically to a superior racehorse that was fraudulently entered in a race under another horse's name or a fictitious name in order to obtain better odds in the betting.
The racing term may derive from the practice of substituting brass for gold in rings, thus leading to the use of ringer as a term meaning "imposter" or "fake." In any case, it appears that when the racing term was coined in the late 1800s, the deception also involved disguising and substituting a superior horse that was certain to win a given race.
In horse racing, the first three horses to cross the finish line are in the money, and it was customary for sportswriters to only write about those three prizewinners—giving their times and other details of interest—and to only mention the horses that also ran in the race at the end of the report.