This is a candidate or an idea that is put forth to “test the waters” and see how something is received, while hiding the mastermind behind it.
It’s named for a fake horse that hunters would use when stalking prey. The logic is subtle: “stalking horse” means “a horse used for stalking,” not “a horse that is stalking” (remember, horses don’t really stalk anything).
So, the “stalking horse” is the thing you hide behind, or the tool you’re using to get achieve your goal.
In finance, a “stalking horse offer” is a pre-arranged bid that effectively acts as a reserve, or lower-boundary on what the seller will receive.