One of the key factors that makes a Mega Man video game so memorable is the continuing onslaught of Dr. Wily's evil robot masters. Except for the occassional exception, each traditional game in the series consists of eight humanoid-based robot masters that Mega Man must destroy in order to reach the final set of fortress levels. Each robot master commands a specific weapon that, when defeated, is acquired by Mega Man. Each robot master is susceptible to another robot master's weapon. Note that while there are many other robots in the Mega Man world, a robot is only a robot master if Mega Man can defeat him and take his weapon.

While the majority of the robot masters were designed by Capcom staff, several of them were created by fans of the series in the then-annual Robot Master Design Contest. Japanese gamers were invited to pitch suggestions for Mega Man 3 through Mega Man 5 for the NES, while American gamers had a chance to send in submissions for Mega Man 6. While some of the robot masters are rather well known and have appeared elsewhere (Cut Man, for instance), others have made their single appearance and faded into history (when is the last time you heard from Yamato Man?). Below is a listing of all robot masters and the games in which they appear.

Mega Man (1987 Nintendo Entertainment System, later ported to the Sega Genesis in 1993, the Sony PlayStation in 1997, the Nintendo GameCube and Sony PlayStation 2 in 2004)

Mega Man 2 (1989 Nintendo Entertainment System, later ported to the Sega Genesis in 1993, the Sony PlayStation in 1997, the Nintendo GameCube and Sony PlayStation 2 in 2004) Mega Man 3 (1990 Nintendo Entertainment System, later ported to the Sega Genesis in 1993, the Sony PlayStation in 1997, the Nintendo GameCube and Sony PlayStation 2 in 2004)

Mega Man 4 (1991 Nintendo Entertainment System, later ported to the Sony PlayStation in 1997 and the Nintendo GameCube & Sony PlayStation 2 in 2004)

Mega Man 5 (1992 Nintendo Entertainment System, later ported to the Sony PlayStation in 1997 and the Nintendo GameCube & Sony PlayStation 2 in 2004) Mega Man 6 (1993 Nintendo Entertainment System, later ported to the Sony PlayStation in 1997 and the Nintendo GameCube & Sony PlayStation 2 in 2004) Mega Man 7 (1995 Nintendo Entertainment System, later ported to the Nintendo GameCube and Sony PlayStation 2 in 2004)

Mega Man 8 (1997 Sony PlayStation and Sega Saturn, later ported to the Nintendo GameCube & Sony PlayStation 2 in 2004)

Mega Man and Bass (1998 Super Famicom, later ported to the Game Boy Advance in 2003)
This game includes a robot master database in which all robot masters created up until this point are listed. Mega Man: Dr. Wily's Revenge (1990 Game Boy, later ported to the Game Boy Advance) Mega Man II (1991 Game Boy, later ported to the Game Boy Advance) Mega Man III (1992 Game Boy, later ported to the Game Boy Advance) Mega Man IV (1993 Game Boy, later ported to the Game Boy Advance) Mega Man V (1994 Game Boy, later ported to the Game Boy Advance) Mega Man (1990 PC) Mega Man 3 (1991 PC) Mega Man (1994 Game Gear) Rockman and Forte (1999 Wonderswan) Mega Man Soccer (1994 Super Nintendo Entertainment System)
This is not a traditional Mega Man game. It is a soccer game with characters and robot masters from the series. It's also one of the only game in the series that allows you to play as a robot master. Rockman Battle and Chase (1997 Sony PlayStation)
This is a Super Mario Kart clone that features Mega Man characters. Mega Man: The Power Battle (1995 Arcade, later ported to the Neo Geo Pocket Color in 2000 and the Nintendo GameCube & Sony PlayStation 2 in 2004). Mega Man 2: The Power Fighters (1996 Arcade, later ported to the Neo Geo Pocket Color in 2000 and the Nintendo GameCube & Sony PlayStation 2 in 2004). Years after Mega Man's legendary battles a new breed of robots were constructed based on Dr. Light's designs for a robot capable of making its own decisions. The prototype, Mega Man X (known simply as X), was discovered by Dr. Cain in the early twenty-second century. Using X as a blueprint, Cain constructed a series of animal-based robots known as Reploids. Unfortunately, over time some Reploids decided that humanity was an obstacle to an all-Reploid society and they began to exterminate humans. Dubbed "Mavericks" by the remaining Reploids, X and his Reploid buddy Zero were dispatched to defeat the Mavericks and their leader, Sigma.

Mega Man X (1993 Super Nintendo Entertainment System, later ported to the PC in 1994)

Mega Man X2 (1994 Super Nintendo Entertainment System) Mega Man X3 (1995 Super Nintendo Entertainment System, later ported to the PC, Sony PlayStation, and Sega Saturn in 1996) Mega Man X4 (1997 Sony Playstation, later ported to the PC in 1999) Mega Man X5 (2000 Sony PlayStation, later ported to the PC in 2002)
For some bizzare reason, Capcom's translators renamed the Mavericks to names based on members of the band Guns 'n Roses for the American version of the game. The first name is the "real" name, the second is the tribute name. Mega Man X6 (2001 Sony PlayStation) Mega Man X7 (2003 Sony PlayStation 2) Mega Man Xtreme (2001 Game Boy Color) Mega Man Xtreme 2 (2002, Game Boy Color) Other spinoffs from the original Mega Man series handle acquiring weapons somewhat differently. The Mega Man Battle Network series features various robots based on the robot masters, although the weapons themselves come from the acquisition of battle chips. Mega Man X's spinoff, MegaMan Zero, relies on gaining experience and items to further Zero's weapon abilities; defeating bosses merely clear levels and do not award specific weapons or techniques. It would seem that the Mega Man universe is branching out and leaving the role of the traditional robot master behind.

* Although this robot master appears here in a cameo, Mega Man cannot take his weapon.


References:
http://www.mmhp.net
http://www.GameFAQs.com
Based on a writeup by PaSTE