Within an earlier, unreleased build of Fuckboy's 3, one can fight against an enemy that is cut from the final game: the Endoskeleton Core!
This boss is pretty much just an earlier version of the 90's Endoskeleton boss fight, lacking the Atari Land portal and instead using a door while overall being somewhat of an easier fight.
For the most part, it shares all of the same skills with 90's Endoskeleton. However it does somewhat differ in the sense that it only has a resistance to Paralysis and Sleep, while the 90's Endoskeleton has a 10% chance of inflicting Poison with an attack and being able to move twice.
It does also possess a few differences when it comes to the HP and ATK, since it has 180000 HP compared to the 90's Endo's 130000 and having 650 ATK instead of 550; meaning that while it can only move once unlike its final counterpart, it can hit way harder.