I found the concept art you based this on.
https://media.discordapp.net/attachments/646509151119474710/922987275929923645/level4.jpg
The big Peashy seems to have been the planned Final boss during the 2003 era of Peashy,i designate the stages of development into eras depending on how much different they were from the final game.
I can try to find the other concept art so...just ask me if you want them.