“Zombies Ate My Neighbors” is a 1993 run and gun ‘horror’ video game for the Sega Mega Drive/Sega Genesis and Super NES platforms. This video game was originally developed, distributed and produced by LucasArts as a comical tribute to both classic and schlocky horror films of the 1950s through the 1980s. [Infos from Wikipedia]
It seems that a beta version (titled Monsters) was leaked somehow online and through a password (BCDF) you are able to play a bonus level that was later removed from the final game (the BCDF bonus level in the released game is different from this one). Thanks to a video from WillowAlchemist Youtube Channel we can see this beta area. Another beta level (he Son of Dr Tongue Bonus Level) was found by Superfun64: it has no monsters, is smaller, and has different music.