Where did the idea come from? Well, in 1968, Toho made a film called Daikaiju Shoshingeki, Americanized later to Destroy All Monsters, in which aliens control ten monsters (Godzilla, Anguirus, Rodan, Varan, Mothra, Manda, Baragon, Gorosaurus, Minya, and Kumonga) on Monster Island to destroy major cities. However, control is broken, and the humans control the monsters for use against the alien controlled King Ghidorah, who, judging from past movies, cannot defeat two or three monsters, let alone ten (or seven rather, since Manda, Varan, and Baragon just sat there, but the fact remains the same).
In 2002, Atari made a Godzilla game for Gamecube based on this concept. Aliens control all the monsters to destroy places, except one (the player) who has to defeat them all. The game was Godzilla: Destroy All Monsters Melee.
First, there's the obvious fact: the monsters are completely different.
Varan, Manda, Baragon, Gorosaurus, Minya, Kumonga: These guys don't even show they're faces. Maybe this is a good thing because for the most part, they're basically just wusses (especially Minya). Varan had appeared in Godzilla: Monster of Monsters (NES). Baragon was in Godzilla: Monster of Monsters, Godzilla 2: War of the Monsters (NES), and Godzilla (GB). Minya was in Godzilla.
Godzilla: Atari put in two Godzillas: Godzilla 90's and Godzilla 2000. Godzilla 90's is the Godzilla that stomped around in the Heisei timeline. Godzilla 90's is also the result of really bad things done by time-borking in Godzilla vs. King Ghidorah (1991). Thus, it could be considered a different version of the same Godzilla from the two earlier Heisei movies, Godzilla 1985 (1984) and Godzilla vs. Biollante (1989). Godzilla 2000 is from yet another timeline: the one depicted in Godzilla 2000 (1999). This probably explains why Godzilla 90's and Godzilla 2000 don't ever fight in Adventure mode. Godzilla had appeared in Godzilla: Monster of Monsters , Godzilla 2: War of the Monsters, Super Godzilla (SNES), Godzilla: Kaiju Daikessen (SNES, Japanese), and Godzilla.
Anguirus: Anguirus is probably the only element from Destroy All Monsters that is still around. Anguirus does all the stuff he usually does, with the exception of a Sonic Roar "beam" weapon and his rage move, Energy Spikes. The Anguirus on the game is otherwise unchanged (for the most part) from the one in the Showa series movies, probably since he hasn't been seen since he got his butt whupped by a disguised Mechagodzilla in Godzilla vs. Mechagodzilla (1974). Anguirus was also in Godzilla: Kaiju Daikessen and Godzilla.
Megalon: I don't even know why they put Megalon in the game. Megalon is stupid. Anyway, Megalon was not in the original Destroy All Monsters movie, since his debut (and only) film, Godzilla vs. Megalon (1973) occured after Destroy All Monsters was filmed. He is pretty much unchanged from the movie, except for the fact that his Magnetic Vortex move just seemed to come out of nowhere. Megalon has been in Godzilla: Kaiju Daikessen.
Gigan: Gigan is one of the more popular monsters out there, so it didn't surprise me when I saw him on Destroy All Monsters Melee. Like Megalon, Gigan (sadly) wasn't in Destroy All Monsters, since his debut movie, Godzilla vs. Gigan (1972), was filmed afterwards. Godzilla vs. Gigan made him look pretty cool, but then it all went down the toilet in Godzilla vs. Megalon whe he fought alongside Megalon. He acted like a frickin' Power Ranger. However, he seems to have redeemed himself in his other game appearances in Godzilla: Monster of Monsters and Godzilla: Kaiju Daikessen.
King Ghidorah: Apparently, Ghidorah is mind controlled in the first place, instead of just a last ditch effort by the aliens near the end. For some reason, Ghidorah looks like he did in Godzilla vs. King Ghidorah, but retains his roar from the Showa series movies he was featured in. Ghidorah also is one of the few who can fly (obviously). Ghidorah was also featured in the games Godzilla: Monster of Monsters, Godzilla 2: War of the Monsters, Super Godzilla, Godzilla: Kaiju Daikessen, and Godzilla.
Destoroyah: Featured in the last Godzilla Heisei film, Godzilla vs. Destoroyah (1995), Destoroyah is put in this game for some reason. Apparently, a lot of people liked Destoroyah in Japan. However, the Heisei series movies (with the exception of Godzilla 1985) never made it to American movie theaters, so why did the designers put in Destoroyah if only dedicated G-Fans knew who he was? Simple. The majority of the people buying the game were dedicated G-Fans anyway (ie myself, Shin-Goji, C.L. Werner, and Pooto, who only plays my copy of the game). Destoroyah can't even fly in the game. He flew in the movie, so why not let him fly here? Destoroyah was not featured in any other games.
Rodan: Rodan was sort of another Destroy All Monsters element added in the game, but it isn't the same Rodan. Judging from the Uranium Heat Beam, he is really Fire Rodan, another version of the Heisei series Rodan from Godzilla vs. Mechagodzilla II (1993). However, his coloring is not that of Fire Rodan, but just of regular Rodan. Rodan was also in Godzilla 2: War of the Monsters and Godzilla.
Mecha-King Ghidorah: Mecha-King Ghidorah is supposed to be regular King Ghidorah revived with cybernetic parts. In Adventure mode, however, you use him to fight King Ghidorah and vice versa. Also, if you're another monster, you have to fight both Ghidorah and Mecha-Ghidorah. Mecha-Ghidorah was also featured in Super Godzilla.
Mechagodzilla: My favorite monster! The weird thing is, the Showa Mechagodzilla from Godzilla vs. Mechagodzilla and Terror of Mechagodzilla (1975) is built by aliens, but this Mechagodzilla is from the Heisei timeline (Godzilla vs. Mechagodzilla II, to be exact) and is created by the UNGCC (United Nations Godzilla Countermeasures Center), aka G-Force, thus is made by HUMANS, not aliens. However, the aliens use him to replace King Ghidorah as the main boss (you can unlock him anyway). It would be cool if you were able to attach the Garuda to his back, or if he could use his prism breath beam (Mega Buster). G-Force Mechagodzilla (the one I was just talking about) was also featured in Godzilla: Kaiju Daikessen.
Orga: Orga is the super secret unlockable character from Godzilla 2000, and I don't know anything about him (on the game), but he'd better use his laser, or I'll be mad. Orga was not featured in any other games.
Mothra: Another retained aspect from Destroy All Monsters, Mothra makes her appearance as well. Once again, they used the Heisei version of Mothra, since she uses her antenna ray. Mothra also appears as an adult (unlike Destroy All Monsters, when she was a larva). One thing that really sucks: Mothra is NOT a playable character! She appears when you pick up a power-up item and attacks your opponents. Mothra was also in Godzilla: Monster of Monsters, Godzilla 2: War of the Monsters, and Godzilla: Kaiju Daikessen.
Hedorah: Hedorah (known by most as the Smog Monster) was not in Destroy All Monsters since, like Gigan and Megalon, his debut (and only) film, Godzilla vs. the Smog Monster (1971) was filmed before Destroy All Monsters. Hedorah would have made a cool playable character, but he just flies around randomly in his "Space Turd" mode and "farts" sulfuric acid mist everywhere, making the players' energy bars replenish slower. Hedorah was also featured in Godzilla: Monster of Monsters, Godzilla 2: War of the Monsters, and Godzilla.
Moguera (Mysterian Mining Robot) or G-Force MOGERA (Mobile Operation Godzilla Expert Robot Aero-type): Moguera/MOGERA has heat rays. Who doesn't want heat rays?
Mothra (Regular, Rainbow, Aqua or Armored): Every other game Mothra was in (sans Godzilla 2), she was a playable character, and she rocked. In Godzilla: Monster of Monsters, she was the other playable monster (besides Godzilla) and she could move more than any other monster (6 spaces!!!). In Godzilla: Kaiju Daikessen, she was the only monster who could fly around and get cheap with her antenna beam! Why couldn't she be playable here? Armor Mothra would be the poop!
Baragon: Besides the SNES games, Baragon was in every single Godzilla game created (The ones I know about, at least). His horn can blind people, he has fire breath, and he could possibly use a dig move similar to Megalon's. Another monster left behind.
Hedorah: Hedorah could stomp around in his "Super Poop" form and knock the bejeezus out of everyone else! Hedorah would have made an awesome character (playable, anyway), that's why he was left out.
Jet Jaguar: Come on! He's Jet Jaguar! They could fudge a beam weapon in for him! He was one of the three of Godzilla vs. Megalon's redeeming factors (the other two being Gigan and Godzilla's flying kick).
Mechagodzilla (Showa): I don't know why they left HIM out! They probably just wanted to make room for G-Force Mechagodzilla. In Kaiju Daikessen, both Mechagodzillas were there! Why not now?
King Seesar: King Seesar has an awesome reflecty move. It's a shame he hasn't been in a single Godzilla game yet!
Biollante: Biollante was in Kaiju Daikessen, so why not put her in this game? Okay, I admit, Biollante sucked in Kaiju Daikessen, but I know you can do a better job, designers! Don't turn your back on her!
Battra: Would do the same stuff as Mothra, but Battra's cooler than Mothra, anyway.
Super Mechagodzilla: You've got G-Force Mechagodzilla, now just put the Garuda on his back. That wasn't so hard, now was it. Come on! It's just better flying and two maser cannons!
Space Godzilla: Actually, Space Godzilla would be a great Godzilla ripoff!
Death Ghidorah: He spews balls and streams of fire! What could be better?
Megaguirus: I don't know much about her, but I heard she could kick serious ass with the rest of 'em!
Varan: Varan was a wimp and could do nothing. Okay, so he could glide. Whoop-de-frickin'-do.
King Kong: King Kong in the Toho movies had a cool lightning grip, but everything else about him sucked. Atari would have probably been sued anyway.
Manda: He can't do anything.
Dogora: He ate carbon and was killed by bee venom. Yep, this would have been the worst game character ever. Worse than Varan (that's a scary thought).
Frankenstein: He relied on brute strength alone in his Toho appearance. Atari would have been sued (again).
The Gargantuas (Sanda and Gaira): They also relied on brute strength. Yawn.
Ebirah: A giant lobster! Don't make me laugh!
Gorosaurus: Many of his aspects would have probably ripped off Anguirus (except for the kangaroo kick).
Mechanikong: NO!
Minya: He has not been shown in a movie since 1969 FOR A REASON!
Kamacuras and Kumonga: Like the others, they can't do anything.
Gabara: He never really even existed.
Titanosaurus: He does have some pretty powerful moves, but he's too stupid looking (and sounding) to be in this game.
Baby/Little/Junior Godzilla: Baby: too small. Little: too weak. Junior: nothing against him, but he's too much like Godzilla.
Dagarah: He's so crappy that I know nothing about him.