Monster Party is...well. It's one of those games that really doesn't lend itself well to description, but in one word, it's weird.
The Japanese version (that went unreleased) contains a wide variety of zany bosses that are blatantly based off of famous movie characters. Because of legality reasons, the bosses were modified for the American release resulting in an even more absurd game than the original.
For two bosses in level one, the localizers simply blacked out the background graphics in the CHR when removing the copyrighted references. All that remains in the US ROM are the tile and collision data.
Round 1 Room 1
Get a better image illustrating that the data still remains.
The localizers edited the speaker and microphone graphics to remove the Little Shop of Horrors reference. Because the localizers never touched the collision data, Mark seems to float in midair where the speaker was removed.
Round 1 Room 4
Tile data for the Statue of Liberty still remains after the Planet of the Apes boss was changed to a happy Pumpkin-Ghost boss.
Unused Controller Code
In the prototype, it greatly increases the time Mark spends in Monster Mode during his first transformation. To trigger it, hold Up + Down + Left + Right + A + B on Controllers 1 and 2, the easiest way being to hold Up + Left + A + B on the former and Down + Right on the latter.
The game accepts 8 passwords in the following format, where # is a digit from 0 to 7 (periods are blanks/skulls):
These passwords will start you on the numbered stage + 1 (so 055 0 would start you on Stage 1, etc.)
A little wolf guy in a race car, found among the Round 1 enemy graphics.
An unused frame for one of the enemies in Round 1.
It appears the crocodiles in Round 4 were going to be able to launch their tails at you like the ones in Round 2.
Some sort of wardrobe, likely meant to accompany the other possessed furniture in Round 6.
This musical note is stored with the graphics for the Round 1 plant boss and is used as projectiles for the boss in the Japanese prototype.
These notes are stored with the punk rocker boss of Round 4. In the final game, he only shoots eighth notes at you.
What appears to be an unused boss. This is stored in the tileset used by the boss rooms in Rounds 4 and 6.
Try to identify all of these.
A series of unused powerups, made to resemble those from popular arcade games of the time. Unused code reveals that the game once utilized an incremental, item-based powerup system (similar to that of arcade shooters like Raiden) for Bert's shots, as opposed to the stage-based powerup system used in the final version.
A cheeky monkey and bone projectile, stored with the other Round 1 boss sprites. These were likely intended to appear during the battle with the Planet of the Apes-inspired boss seen in the Japanese prototype.
It's...a key. While you do receive keys after defeating certain bosses, you never actually see them as collectible items and the key graphic in the status bar looks very different than this sprite.
A bunch of totem pole faces for a scrapped boss that's only present in the Japanese prototype.