Codes
Both codes are entered during the game's main menu, and a noise will play to confirm when they are entered correctly.

Unlock Super Mechagodzilla for Versus:
Hold L + R and press Y, X, A, B

Unlock Gotengo, and its stage, for Versus:
Hold L + R and press Up, Left, Right, Down, Y, X, B, A

Fight Super Mechagodzilla in Normal Mode
To play against Super Mechagodzilla in the Normal, story, mode the player has to play the game on the normal/expert difficulty and get over 1,000,000 points by the final stage.
Beating the game on normal, under these conditions, will display this screen, with the above code.

Fight the Gotengo in Normal Mode
To play against the Gotengo in the Normal story mode the player has to play the game on the expert difficulty and get over 1,000,000 points by the final stage, which will occur after facing Super Mechagodzilla.
Beating the game on expert, under these conditions, will display this screen, with the above code.

AI Player in Versus Mode
To play against the computer in Versus mode, simply move player 2's selection over the preferred opponent and then unhook the second controller from the system. This will cause the game to auto select the highlighted character and when the match starts player 2 will be replaced by the computer.

Earth Cameo
A cleverly hidden easter egg, the battleship Earth, from the Monster Planet of Godzilla ride, will show up briefly in the background if certain conditions are met. First, the match must be Godzilla against Mothra. Secondly, Godzilla must defeat Mothra exactly at the moment that the timer reaches 30 seconds. If done correctly, the ship will fly by while the victory screen comes up, as seen here.