Met is a recurring enemy in the Mega Man series. Their names were taken from the word, helmet- due to their helmet being their greatest defense. Many iterations of these enemies were created throughout the entire Mega Man series.
The Mets are most known for their numerous appearances in the Classic series. In these games, Mets generally hide under their hard hats until Mega Man draws close; then, they either fire a 3-way shot from under their hat (Mega Man 1 and 9) or come out of hiding and move around. Their main feature is their helmet, which reflects shots from the Buster Arm.
Mega Man 9 introduces the fake 1-Up. Much like regular 1-Ups, they are represented by Mega Man's mugshot. However, when the Blue Bomber moves to pick it up, it is revealed to be a Met, who then promptly attacks the hero.
Mets are often seen in other amusing instances throughout the Classic series, including a gumball machine-esqe contraption that infinitely spawns Mets, and behind the controls of a giant bulldozer.
Mega Man ZX features a Met-themed amusement park; the midboss of this stage is appropriately a gigantic Met. Eerily enough, it is revealed that Aile's parents were killed at said amusement park.
The Mets appear in both the MegaMan Battle Network and Mega Man Star Force franchises (as Mettaur and Mettanna, respectively). In these games they are always used in the opening tutorial and, along with the given game's take on the "stationary cannon" concept, are the only viruses to populate the first Internet/Wave Road area that the player enters.
They follow the same pattern in every game: one Met follows Mega Man's row or column (depending on series) until they draw level with the Blue Bomber, after which they attack with a shockwave from their pickaxe. When more than one Met is on the field, they each wait for their turn (hiding under their helmet at higher levels).
The BattleChips that they drop usually involve their shockwave or a shield; generally speaking, they are low-level chips that quickly become worthless to the player after the first boss.
Unlike the other series, where Mets are dedicated to turning Mega Man into a pile of smouldering scrap metal 100% of the time, Mets are implied to be simply instinct-driven and occasionally benevolent in the Battle Network series. Roll can charm Mettaur and make them non-aggressive (although she proceeds to sic them on MegaMan in battle) and MegaMan at one point even defends a herd of Mettaur from a Navi who wants to delete them for kicks.