Whoever first made the observation - some say Winston Churchill, others Ronald Reagan - there is something about the outside of a horse that is good for the inside of a man, and a woman. On stage these noble beasts have inspired some highly successful plays, including Peter Schaffer's Equus, recently revived with Daniel Radcliffe, and War Horse, still living up to its name in the West End after over two years (panto horses probably don't count). In cinema, the legacy is more mixed; but not for nothing is the Western, arguably the greatest of film genres, also known as the horse opera. Horses, Liz Mermin's intensely strange documentary, is a fascinating addition to this stable.