According to "Brewer's Dictionary of Phrase & Fable", a Stalking Horse is a mask to conceal some design, or a person put forward to mislead, or a sham. It is often used in politicial parlance; i.e. a stalking-horse candidate is put foward to mask the real canidate - the stalking-horse withdraws at the last minute. Apparently this comes from sportsmen who would conceal themselves behind horses and go stalking step by step until they got within shot of the game.
Hope this helps! Age.