A candidate put forward in an election to conceal an anonymous person’s potential candidacy. If the idea of the campaign proves viable, the anonymous person can then declare their interest and run with little risk of failure.
A stalking horse candidate is also sometimes used to divide the opposition in order to help another candidate.
Daryl Lyman: “The expression originated hundreds of years ago in old English hunting practices, especially among fowlers. Many kinds of game that would flee at the first sign of humans would not be alarmed by the approach of a horse. Therefore, fowlers trained horses to serve as covers during hunting.”