Bertrand du Guesclin ( c. 1320 â `` 13 July 1380 ) , was a French knight from Brittany who fought in the Hundred Years ' War . Bertrand du Guesclin ( c. 1320 â `` 13 July 1380 ) , known as the Eagle of Brittany or the Black Dog of BrocÃ liande , was a Breton knight and French military commander during the Hundred Years ' War .