The game introduced two new monsters not seen in any Godzilla movie: Obsidius and Krystalak, who was exclusive to the Nintendo Wii version of the game. Further newly created monsters, Fire Lion, The Visitor, and Lightning Bug were also supposed to be featured, but ended up being cut.
The developers originally had a larger monster roster in mind. King Kong was meant to appear, but his inclusion was hindered by licensing issues, as he is not a character owned by the Toho Studio. Zilla, the American Godzilla re-imagining from Godzilla (1998) was also considered, but was eventually dropped due to his unpopularity with American and Japanese fans. Hedorah, the Smog Monster was set to appear as well, but his slimy look and his transformation ability proved to be too difficult to program, so he too had to be cut.
Though he is uncredited, the roars and other noises made by Krystalak and Obsidius were taken from stock monster sound effects originally recorded by Frank Welker some time in the '70s-80s. Incidentally, he also provided additional sound effects for Godzilla (1998).
Dennis Bateman uses the same voice for Spy when voicing General Gyozen. Both Team Fortress 2 and this game came out 2007.
Interestingly, King Ghidorah and his cyborg version Mecha-King Ghidorah in this game are not based on their appropriate Godzilla vs. King Ghidorah (1991) design, but rather the version from the film Godzilla, Mothra and King Ghidorah: Giant Monsters All-Out Attack (2001), which was never actually turned into Mecha-King Ghidorah.