MOGUERA is known as MOGUERA 2 in this game, referencing to "Godzilla vs. SpaceGodzilla", where the first MOGUERA was destroyed.
Contributed by Frazzle
The plant monster Biollante was intended to be a playable character, but was cut late in development. According to Simon Strange, one of the developers, she was complete, but perceived as being "behind schedule", and the final version had not been cleared with the license holder, Toho. She still exists within the game's code, but was made inaccessible.

"I believe she was cut for political reasons, probably stemming from the bad blood that developed between Pipeworks and Atari" - Simon Strange
Contributed by TheBowserPrime