Dark Man is a series of four robots created by Dr. Wily, one of which was created to frame Proto Man, taking his form. They appear in Proto Man's Castle, each occupying their own individual stage in Mega Man 5.
Dr. Wily created the Dark Men intending on getting his revenge on Proto Man for spoiling his last plan. He gave each Dark Man a different ability. The first one can roll around and shoot, the second one can create electric barriers, the third one can freeze the target and shoot like a machine gun, and the last one being able to shoot, use electric barriers like the second Dark Man robot, and shape-shift. The last Dark Man disguised himself as Proto Man to frame him for his crimes, attacking the city with his army of Robot Masters. He kidnaps Dr. Light, leaving his scarf behind, making Mega Man believe that Proto Man became evil. Mega Man defeats the Robot Masters and goes after him to his castle. There, he fights the first three Dark Men, and finally meets with the fake Proto Man. Mega Man refuses to fight, and the fake shoots him, draining his HP to near-death. It's then that the real Proto Man appears and unmasks the Dark Man, and gives Mega Man a L-Tank to recover his health before fighting Dark Man. Mega Man fights him and defeats him, discovering that it was all another plan of Dr. Wily.
The Dark Man robots have a cameo appearance in Mega Man 10's first Wily Castle stage, specifically in the paths between the first and second Weapons Archive, but only their heads are visible. Two can be seen during the spike drop, one occupies another wall, and the other can be found in the floor. As this area of the stage is filled with piles of scrap metal, it can be assumed that the Dark Men were disposed of at some point after Mega Man 5.