It is 36 years since one of the world's greatest racehorses won racing's premier steeplechase, the Cheltenham Gold Cup, for an unprecedented third time. But even at the start of the 21st century, this one horse has - uniquely in the racing world - retained truly mythical status. If people know the name of just one racehorse, it is likely to be that of the courageous Irish horse named after a Scottish mountain: Arkle.