Yellow Devil

Yellow Devil (イエローデビル), also known as Rock Monster, is one of Dr. Wily's creations, that originally was a fortress boss in the first Mega Man game as a big yellow blob made of nanobots, that can dismantle himself and cross through the screen to attack enemies. His weak point is his eye, which is the controller of the nanobots and can shoot small projectiles.

In the remake Mega Man Powered Up, it is revealed that he can talk, but the only thing he can say is "Bumo".

In the arcade games Mega Man: The Power Battle and Mega Man 2: The Power Fighters, he appears as the penultimate boss of the courses "Mega Man 1~2" and "Mega Man 3~6" from Power Battle and "Rescue Roll!" from Power Fighters. He has new attacks like shooting beams from his eye, separating into smaller versions of him, walking, jumping, throwing part of his body that bounces across the room, and absorbing players from his arm to launch them.

Yellow Devil is one of the three enemy racers (the others being a Roader and Sniper Joe) in Mega Man: Battle and Chase, and there is a secret boss recolor of him called Black Yellow Devil (ブラックイエローデビル), sometimes shortened to Black Devil, which caused confusion between it and the Shadow Devil.

He has also a short appearance in the opening scene of Mega Man 8.

Yellow Devil Mark2
An upgraded version of the Yellow Devil called Yellow Devil Mark2 (イエローデビル Ｍｋ－Ⅱ) appeared in the second Wily Castle stage from Mega Man 3. MK2 acts similar to the original, but it can not only do the "blocks moving pattern", but can also flow in a wave. Its weakness is the Hard Knuckle.

Other variations
The Yellow Devil reappeared in several other games in different forms, like: Mega Man 8 and Mega Man and Bass as Green Devil, Mega Man V as Dark Moon, Super Adventure Rockman as New Yellow Devil, Rockman and Forte: Challenger From the Future as Grey Devil, and in Mega Man 9 as "Twin Devil" and enemies called Petit Devils.

The Devil series also appeared in other Mega Man series: Mega Man X5 as Shadow Devil (including a remix from the fortress boss music of the first Mega Man game, its weakness to an electric weapon similar to Thunder Beam, and others), in the first two games from the Mega Man Zero series as Rainbow Devil, and in Mega Man ZX as a miniboss made out of magma named Lava Demon.

Trivia

 * Though the final boss of MegaMan Battle Network 3, Alpha, is similar to the Devils, it's not related to them.
 * In the popular game, I Wanna be the Guy, the Mecha Dragon appeared as the seventh boss, and had an attack where it splits into several cells and moves to the other side of the stage. This is a nod to the Yellow Devil's attack pattern.
 * The music that plays during the Yellow Devil battle in Mega Man: The Power Battle and Mega Man 2: The Power Fighters is a remix of the music from Dr. Wily first two stages in Mega Man.
 * In the U.S., Yellow Devil's name was changed to Rock Monster due to a policy Nintendo had against religious references in NES games at the time.