In the very earliest stage of production, what would become BUG! was supposed to be a Sonic the Hedgehog sequel. Realtime Associates and Sega of America were in the final stages of approval until Sega of Japan learned of it and nixed the project. The game was retooled as an original IP so that the Japanese side couldn't interfere.
According to Playing at the Next Level: A History of American Sega Games, the game was originally developed for the Genesis.