In the first game, they were stationary, but they started becoming mobile in MM2. They've continued to evolve steadilly ever since. By MM3, some of them had appeared that could fly, as well as gigantic Mettaurs as well. MM4 introduced underwater Mettaurs, as well as a gargantuan Mettaur as a boss in Wily's fortress. MM6 also had a Mettaur as a boss; this one was a normal-sized one at the controls of a huge battle mech.
These critters are possibly one of the biggest mascots of the series, and they've even appeared outside the orignal series as well. They've also showed up in all of the X games, as well as the Battle Network series.
There was even a Mettaur that made a small cameo on the TV show. Its name was Dot, and rather than being an enemy, it was actually on of Dr. Light's robots. It only appeared in the first episode, however, and has since vanished.
MM1 introduced us to Sniper Joe. They were well-protected by their shields, but were vulernable to attack when they jumped or fired. Actually, this became a rule for dealing with nearly all kind of Joes. They returned in MM2, this time sporting a red hue and at the controls of a powerful hopping mech.
MM3 produced a variation called Hammer Joe. Much bulkier than the original version, this type carried no shield, but it didn't need it due to its thick armor. It swung a ball and chain that it would throw periodically. Before doing so, it will open its eye, its one weak spot. Another incarnation, appearing in MM4, was Skeleton Joe. This Joe seemed flimsy, as it would fall apart from the weakest attack. But, it had the power to reassemble. The only way to stop that from happening was to totally destroy it with a powerful enough attack.
Two more Joes appeared in MM5. Crystal Joe created hard gems that it'd throw, protecting itself in a forcefield while doing so. Apache Joe piloted a small helicopter. In MM6, Joes appeared in a few places behind the controls of a mounted turret. MM7 reintroduced the Sniper Joe in his former glory. MM8 and MM&B also had Snipers, but these ones had hand grenades in their arsenal as well.
In MMX, there was a green-colored, shield-carrying reploid that fired a spiked ball, no doubt inspired by the Joe design.
Battontons first appeared in MM2, and their design has changed very little since then. MM8 gave us a couple of modifications. One was a type that sprouted a drill and tried dropping onto its foes. Another kind had large fangs and would use them to grab onto its prey.
A single Battonton made a cameo appearence in MMX, showing up at the bottom of the long shaft at the beginning of Armored Armadillo's stage. It always dropped a 1-up when defeated, and it respawned when you left the screen and returned, so it became a great place to rack up lives quickly.
Battontons have also made frequent appearences in the TV series, more so than any other stage enemy. They only appeared in a single episode in Season 1, but in Season 2, they became a more major component in Wily's forces, showing up in numerous episodes.
This monstrosity first appeared in MM1 as the Yellow Devil. This yellow/orange goo that composed the body was the most common. It attacked mainly by firing beams from the eye, or by disassembling the main body and hurling chuncks of itself. The Yellow Devil returned in MM3, with a slightly different appearence, but the same overall attacks.
In MM5GB, Megaman battled with another Devil creature. Terra called it Dark Moon, but it was actually just a modified Devil Series.
The Yellow Devil returned in The Power Battle, and well as The Power Fighters. This time it had been given plenty of new attacks, such as hurling large bouncing blocks of goop, or splitting into three smaller Devils.
MM8 gave it a color change. It was now the Green Devil. It has different attacks from its yellow counterpart as well. Green Devil reappeared in MM&B, but it was the intro stage boss and was much easier than its other incarnations.
But the Devil Series didn't confine itself to the original series. In X5, a Black Devil appears in the Zero Virus region. This new Devil has very poweful attacks, but it's also quite nostalgic, as the original Wily boss music plays through this fight, and a hazy 'W' appears in the background.
It even showed up in the Zero series. Both Z1 and Z2 had a color-changing Rainbow Devil appear in the later stages of the game. However, this version was smaller than most models.
The first game introduced us to the most infamous of these guys; BigEye. They're very hard to destroy, and can kill you in as few as three hits. Standing your ground against one is not the way to go. You're best off trying to avoid them somehow.
MM2's version of this was a Sniper Joe riding a hopping mech. In addition to causing major damage by jumping on you, it can fire plasma shots, and even after destroying it, you still have to fight the Joe.
Later games had more genaric models, but all of them prove to be some of the most fearsome of stage enemies.