Someone or something whose role is to become the focal point for, or the initiator of, a debate or challenge. However, in reality their leadership role is an illusion, and the stalking horse is really working to promote a challenge or debate that will benefit a third party whose identity remains a secret.
Pete: "You been watching what them Clintons are up to?"
Cook: "I've been trying to ignore them now for 12 years."
Pete: "Well Slick Willie has been attacking Obama."
Pete: "Well he is acting as a stalking horse for Hillary to damage Obama's rep."
Cook: "I figured Obama already damaged his rep."
Pete: "Not as much as Willie can."
Cook: "Your wisdom is boundless."