Okay so, I take it you're a guy who just likes to know everything.
There are 3 "hidden" remix versions of 3 of the bosses the mod adds. They are all absolutely ridiculously stupidly difficult to the point where I'm doubting if people can beat them at all. If you *really*, **really**, ***really*** want a challenge that has no particular reward other than your own feelings about being able to beat it, if you truly believe this is the kind of thing you want to do, then these are for you. If you want an actual in-game reward for beating any of these or you think you're oh-so-good at this game cuz you beat the remix bosses that CDG put in the vanilla game, they probably aren't for you, but I don't judge.

Enough preface, you access them by beating one of the mod bosses (fe, Graveyard Boss) and then _without advancing to the next NG_, revisiting the area and getting back to its boss. Since you already beat the normal version on your current NG, it will spawn in the remix version.
Again, there is absolutely _no_ gain to beating or attempting these bosses other than bragging rights.
But, you know, if you do happen to actually beat one of them, I would be very curious to see a video of your successful fight :-)