Sir Menaduke of Mentoche was a Knight of the Round Table and relative of Sir Lancelot. He, naturally, defected from Arthur's Court with his cousin after Lancelot and Guinevere's affair was uncovered. He fought well at the Battle of Benwick and Joyous Guard, but went into exile in Francia where he was rewarded with the Earldom of Rouerge. He seems to have returned to Britain to help put down Mordred's rebellion but was killed in the fighting.
His name is probably a corruption of the Welsh, Meriadoc, though he does no appear to be the same as either of the Arthurian characters of that name.