The Brown Knight had a castle somewhere in the vicinity of the Duke de la Rowse, and his hobby was slaying other knights and imprisoning their ladies. He had thirty widows captive in his castle. Gareth came by and slew him, liberated the castle and sent the ladies to Camelot.
If he had not been slain, one could have identified him with Breuse Sans Pitie, whose original name was Brun (“Brown”).
Source
Le Morte Darthur | Sir Thomas Malory, 1469-1470