Parody World: Monster Party (found Famicom prototype of "Monster Party" NES game; 1989)

Title Screen.

Footage of the prototype, as originally only posted on Nico Nico Douga.

Monster Party is an NES platform action game setting a kid named Mark in an alternate world, populated with monsters and supernatural creatures, lots of them based on popular culture horror icons or Japanese ghost mythology.

Due to copyright issues, many of the bosses had to be changed, and most of the blood was removed due to the regular Nintendo of America censorship. In an old gaming magazine, two preview screenshots of the game were different from the final product:

One showed the title screen being covered with red blood instead of green slime in the released game, and the selected cursor being a skull instead of a pumpkin; the spider on the left side of the screen also looked more like a humanoid face on legs.

The other shows the Man-Eating Plant boss being different than in the final product: its head is more rounded, looking more like a Piranha Plant/Audrey II, and there is a stilted microphone and a speaker around it. In the released gameplay, the player character can stand on the area where the speaker originally was, despite being empty background.

According to other sources, two of the bosses had to have their sprites changed because they were too similar to licensed characters: a boss looking like a Mogwai in a box from Gremlins has been changed into a cat, and an Ape from Planet of the Apes has been changed into a Pumpkin-headed ghost.

As seen in the recently released Famicom version, which is the original NES prototype shown in the magazine, almost every boss was a movie reference or had a different design. A boss based on Damajin, a Japanese movie monster and a different Medusa boss were among the changed features before the final release of the game.

In the leaked release, it is noticeable that the music which plays after the first level transitions into its gorier form in the NES release is missing in the Famicom release; even earlier versions of the game didn't have the transition at all, and different background tiles of more detailed mountains.

Images from the prototype

title screen
Audrey II
not shown: sprite flicking on the horse and occasionally on the screen
The Medusa boss

UPDATE 4/22/14 http://nintendoage.com/forum/messageview.cfm?StartRow=126&catid=35&threadid=121367 <-- The prototype has been procured from the original owner by Beaglepuss, who on this page on the forum, has said he'll dump the game himself and make it public, "within the year (Hopefully a great deal sooner)".

UPDATE 3/15/15 http://www.nintendoage.com/forum/messageview.cfm?StartRow=251&catid=35&threadid=121367 <-- The prototype was dumped on the date of July 3, 2014.

https://tcrf.net/Proto:Monster_Party has full details on the changes