Hunters, almost since time immemorial, have made use of a variety of tricks and ruses devised to enable them to approach their quarry (or vice-versa) without being noticed.
One of the oldest of these is the stalking-horse.
At first, as the name indicates, it was an actual horse, behind or alongside of which the hunter walked in concealment.
Later, the horse was draped with cloths or the like, and the hunter might be beneath rather than behind the covering.
Either way, the game could see only the horse, and was duped into thinking there was no hazard.
As time went on, the term became used as a general one to describe something used to conceal a secret project, usually a somewhat dishonorable one.
In the political field, it has been employed to describe a third candidate deliberately entered into a race in order to draw votes from the more popular of the original entries with the hope that the less popular would become the winner.