Before I get to the MM42 easter eggs, just a tidbit from the Prometheus development. Is there another game where you fight a firebreathing Chimaera, playing as Prometheus, while riding on a Pegasus? I think not :)
There are two small easter eggs in MM42, and so far nobody has reported that they've found them to me. This means either nobody has actually found them, or they just haven't bothered to tell.
I imagine people might want to try to find the easter eggs by themselves now that I've told about them, so be wary of spoilers below if you don't want to know how to find them.
**SPOILERS**
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How to find the easter eggs: Go to Mad Man's stage. At the very bottom of the downward vertical section there is an Elehead, and you are supposed to go the left into a padded room in order to get to the boss room. Here there is actually an additional path to take.
Kill the Elehead enemy and jump up one step on the stair-looking thingy to the left. Use Launch Missile while facing right and jump onto the missile. When you are at the right edge of the screen jump up to the right and you should enter a hidden boss room. It houses Etc Man, an early scrapped Robot Master. Etc Man spawns an enemy from the 31 available at random as his special attack. In case you can't tell I'm a sucker for randomness, mostly stemming from the fact that I used to play Magic: The Gathering for many years. I'm really pleased with this fight, and the guy deserves to have a stage of his own I think, but since I added him in like a week before the release there was no time for that. The reason Etc Man had sprites I could use was that he was suggested by Skyman and I made a very early animation with Etc Man in it in 2001 as a concept sketch for the game.
If you beat Etc Man and then beat the game you will get a special end screen with Beck from Mighty No. 9 in it :) Enjoy!