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Machine Wars,

A group of twelve Transformers that were released in the US. Many (all?) were second releases of the Canonmasters (=Turbomasters) and Scopemasters (=Predators) in different colors. They were exclusives to Kay-Bee Toys, a single chain of toy stores. Their names are (the names of the Canonmaster/Turbomaster and Scopemaster/Predacon which is the same shape is indicated between brackets):
Hoist
Hubcap
Megaplex
Megatron
Mirage
Optimus Prime (Eagle/Thunderclash)
Prowl
Sandstorm (Rotorstorm)
Skywarp
Soundwave (Buzzard/Stalker)
Starscream (Crash/Skyquake)
Thundercracker
Thanks to Sszl111 who supplied me the info on Machine Wars.

Magnaboss,

Maximal. This is a Beast Wars Gestalt, three Maximals are part of this 3-pack: Ironhide, Prowl and Silverbolt.

Mainframe,

Autobot Actionmaster. His weapon transforms into a service droid.

Manterror,

Predacon. Comes from the Beast Wars. Transforms into some sort of slug.

Marissa Fairborn,

Captain in the Earth Defense Command. Good friend of the Autobots in the post-movie series.

Master,

Homeplanet of the Headmasters in the Japanese series.

Matrix,

Most important item of the Autobots. The Autobot leader is always the bearer of the Matrix. In The Movie it was given to Ultra Magnus by Optimus Prime. At the end of The Movie Hot Rod obtained it and became Rodimus Prime. In The Return Of Optimus Prime, part 2, Optimus Prime got the Matrix back and released its powers to save everyone of the hate plague.

Maximals,

The "good" guys of the Beast Wars. Click HERE to see the names of all the Maximals.

Meanstreak,

Autobot from the second generation. Transforms into a car. He is one of the Powermasters.

Megaplex,

Machine wars. Transforms into a jet. He has the same shape as Megatron.

Mega Pretenders,

Transformers that consist out of two transformable robots, the smallest of the two robots can use the second one as a protective casing. In fact the second robot is just a casing according to the comic books and therefore have no own names. The names of the three Mega Pretenders are:
Crossblades
Thunderwing
Vroom

Megatron,

Decepticon Actionmaster. His tank transforms into a battlestation and a jet.

Megatron,

Decepticon. Transforms into a gun. He was created at the beginning of the Cybertronian Wars, he was one of the strongest Decepticons and soon he became their leader. He learned of a space expedition of the Autobots and he and his most important soldiers followed the Autobot ship; the Ark. He left the command of Cybertron in the hands of Shockwave. Megatron, his Decepticons and the Autobots crashed on Earth and were deactivated for a few milion years. Upon reactivation Megatron put up a new headquarters on Earth and reastablished communications with Cybertron. In The Movie he and the Autobot Leader, Optimus Prime battle each other to the death. Optimus Prime wins but dies soon afterwards from his injuries. Megatron survives, but is also badly injured. He is carried away by Soundwave from the battlefield. Starscream sees in this situation finally a chance to take command from Megatron and he throws Megatron in outer space. There Megatron encounters Unicron and Megatron is rebuilt into Galvatron.

Megatron,

Decepticon from the second generation. I have just heard that he exists, I don't know in what he transforms (probably a car, just like all the gobots that I know). He is one of the Gobots.

Megatron,

Decepticon from the second generation. Transforms into a tank, he is the Decepticon Leader

Megatron,

Predacon. Comes from the Beast Wars. Predator leader, transforms into a Tyranosaurus rex.

Megatron,

Hero Megatron. A generation 2 release of Megatron. Only appeared in the US, it's the same robot as the European Transformer Archforce (see pic there).

Megatron,

See Optimus Primal & Megatron.

Megatron,

Predacon. Comes from the Beast Wars. He is a Transmetal Figure, transforms into a Tyranosaurus Rex or two hoverjets can flip out of his sides

Megatron,

Machine wars. Transforms into a jet. He has the same shape as Megaplex.

Megazarak,

Decepticon. Japanese name for Scorponok.

Melt Down,

Decepticon Micromaster. Transforms together with Half-Track into a military vehicle. He is a member of the Battle Squad.

Menasor,

This Transformer is a Gestalt, formed by the five Stunticons.

Metro Squad,

Autobot Micromasters. This squad is one of the squads of Micromaster Combiners. The group is formed by:
Oiler
Power Run
Road Burner
Slide
Strike Down
Wheel Blaze

Metroplex,

Autobot. Transforms into in major part of Autobot City. His first appearance is in Five Faces Of Darkness. His Decepticon adversary is Trypticon.

Micromaster Bases,

These Micromaster vehicles can transform into bases. Both vehicles have their own commander. The names of the commanders are: Groundshaker and Skyhopper.

Micromaster Battlestations,

These Micromaster buildings can transform into bases. All bases have their own commander. The names of the commanders are:
Airwave
Greasepit
Hot House
Ironworks

Micromaster Combiners,

Four squads of six robots each. Two robots can combine together to form a vehicle, so each squad consists out of six robots, making three vehicles. The names of the squads are:
Astro Squad
Battle Squad
Constructor Squad
Metro Squad

Micromaster Combiner Anti-Aircraft Base,

See Decepticon Anti-Aircraft Base.

Micromaster Combiner Autobot Headquarters,

See Autobot Headquarters.

Micromaster Combiner Transports,

Three sets of Transformers. Each set consists out of two Micromasters and one large vehicle. The two Micromasters transform together into a vehicle. The large vehicle transforms into a base. The name of the three sets are:
Cannon Transport
Missile Launcher
Tanker Transport

Micromaster Patrols,

All the patrols consist out of four Micromasters. The names of the patrols are:
Air Patrol
Air Strike Patrol
Battle Patrol
Construction Patrol
Hot Rod Patrol
Military Patrol
Monster Truck Patrol
Off Road Patrol
Race Car Patrol
Race Track Patrol
Rescue Patrol
Sports Car Patrol

Micromaster Rocket Base,

See Countdown.

Micromaster Space Shuttle,

See Skystalker.

Micromaster Transports,

These Micromaster have their own vehicles or AA-bases. Their names are:
Erector
Flattop
Overload
Roughstuff

Military Patrol,

Decepticon Micromasters. This is one of the Micromaster Patrols. The group is formed by:
Bombshock
Dropshot
Growl
Tracer

Mindwipe,

Decepticon. Transforms into a bat. In The Rebirth, part 2 his head was modified into an exosuit for Vorath. Due to this proces, he became a Headmaster.

Mini-Cassettes,

Group of Autobots that can transform into cassettes, they have Blaster as their guardian. For the Decepticon Mini-Cassettes, see Spy-Cassettes. The names of the Autobot Mini-Cassettes are:
Eject
Grand Slam
Raindance
Ramhorn
Rewind
Steeljaw

Mini-Jets,

Group of three Decepticons from the second generation. They have all been released for a second time, but in different colors. Their names are:
Hooligan, colors: gray or orange
Skyjack, colors: black/red or black/blue
Space Case, colors: green or white/blue

Mini-Vehicles,

A large group of Autobots with very diverse transformations. The following Transformers belong to the Mini-Vehicles:
Beachcomber
Brawn
Bumblebee
Cliffjumper
Cosmos
Gears
Hub Cap
Huffer
Outback
Pipes
Powerglide
Seaspray
Swerve
Tailgate
Warpath
Wheelie
Windcharger

Mirage,

Autobot. Transforms into a race car. He is one of the Autobot Vehicles.

Mirage,

Autobot from the second generation. Transforms into a car. He is one of the Gobots.

Mirage,

Machine wars. Transforms into an Indy 500 racer. He has the same shape as Prowl.

Misfire,

Decepticon. Transforms into a jet. In The Rebirth, part 2 his weapon was modified into a exosuit for Aimless. Due to this proces, he became a Targetmaster.

Missile Launcher,

Autobot Micromaster Combiner Transport. This transport transforms into a base. The base is operated by Retro and Surge. This is one of the Micromaster Combiner Transports.

Missile Master,

Autobot Micromaster. Transforms together with Moonrock into a missile launcher. He is a member of the Astro Squad.

Mixmaster,

Decepticon. Transforms into a construction vehicle. He is one of the Constructicons.

Monsterbots,

Group of three Autobots. Their names are:
Doublecross
Grotusque
Repugnus

Monster Truck Patrol,

Autobot Micromasters. This is one of the Micromaster Patrols. The group is formed by:
Big Hauler
Heavy Tread
Hydraulic
Slow Poke

Monzo,

Nebulon. Transforms into Weirdwolfs head.

Moonracer,

Female Autobot. One of the Autobot rebels on Cybertron in the time before The Movie. She is the girlfriend of Powerglide.

Moonrock,

Autobot Micromaster. Transforms together with Missile Master into a missile launcher. He is a member of the Astro Squad.

Motolasers,

See Lasercycles.

Motorhead,

Decepticon Micromaster. Transforms into a car. He is a member of the Decepticon Race Track Patrol.

Motormaster,

Decepticon. Transforms into a truck. He is the leader of the Stunticons.

Motormouth,

Autobot from the second generation. Transforms into a car. He is one of the Gobots.

Motorvators,

Group of three Autobots. They are all accompanied by a Nebulon. This Nebulon transforms into the face of the Motorvator. These Transformers are a little different from Headmasters: the Nebulon of the Motorvators is placed in the chest of the larger robot, when you close the chest a part of the Nebulon is pushed upwards to form the face. Their names are:
Gripper
Flame
Lightspeed

Mudslinger,

Autobot micromaster. Transforms into a car. He is a member of the Off Road Patrol.

Muzzle,

Nebulon. Transforms into Nightbeats head.