Stalking-horses - four-footed ones - were used from the 16th century onwards to deceive deer or gamebirds. The game didn't see any danger in horses, so hunters hid behind them until they were within easy range before letting the game have it. Some hunters, lacking a horse, rigged up a painted screen to look like one. By Queen Victoria's time the technique was on the way out; and today, with our long-range rifles and telescopic sights, we don't need it. Today's stalking-horse is a political animal.
Tony Marlow suits the original role, being so harmless (if not barmy) in John Major's eyes that he can ignore him; Mr Marlow may snort and neigh, but he is only, as it were, a horse. A false sense of security is thus encouraged, and the attention diverted while the men with the fire-power lurk behind. Some observers, though, would regard Mr Marlow as a painted screen rather than a horse, arguing that to earn the true label he would have to make the formal challenge.
The expression was used in its metaphorical sense from early on in its history, both of people and of deceptive ideas, but its application to people seems to have died out at one time. The editors of the OED were marking it 'obsolete' in 1914. Its revival, not only in politics but also in business, is hard to explain. Could it be that there are more of them about? I don't think so. Someone dug it up and it became a desirable acquisition.Reuse content