Donald Vaughan Sinclair (22 April 1911 – 28 June 1995) was a British veterinary surgeon who graduated from the Royal (Dick) School of Veterinary Studies in 1933. He was made famous as the inspiration for the eccentric character Siegfried Farnon in the semi-autobiographical books of James Herriot (Alf Wight), adapted for film and television as All Creatures Great and Small.