The title Marquess of Dorset has been created three times in the Peerage of England, it was then created in which year, for Edmund Beaufort, 1st Earl of Dorset?
Marquess of Dorset:  It was then created in 1442 for Edmund Beaufort, 1st Earl of Dorset, who was created Duke of Somerset in 1448. Edmund Beaufort, 2nd Duke of Somerset: Edmund Beaufort, 2nd Duke of Somerset, KG (1406 – 22 May 1455), sometimes styled 1st Duke of Somerset, was an English nobleman and an important figure in the Wars of the Roses and in the Hundred Years' War.
1442