Donald Vaughan Sinclair (22 April 1911 – 28 June 1995) was a British veterinary surgeon (graduated Dick Vet 1933) made famous as the eccentric character Siegfried Farnon in the semi-autobiographical books of James Herriot (Alf Wight), later adapted for film and television as All Creatures Great and Small.
In 1939 he bought an existing veterinary practice at 23 Kirkgate, Thirsk, Yorkshire, and in July 1940 hired Wight to run it while he (Sinclair) was undertaking his war service in the Royal Air Force. However, Sinclair had deliberately misrepresented himself as being younger than he was in order to join up, and it was quickly discovered that his reflexes were not fast enough for him to continue with pilot training. He could have been redeployed within the service, but the fact that he was a veterinary surgeon (a reserved occupation) meant that he was considered more useful to the war effort by resuming his peacetime profession. The severe national food shortage meant that proper veterinary treatment of farm animals received a very high priority, and so within four months of joining the RAF he received a compulsory discharge and returned to Thirsk.