Found within the game's files is an unused title theme for Cuphead with lyrics. This may have been composed during early stages of the game's development, as the lyrics describe a completely different plot from what was featured in the final game.

According to Jake Clark, when he was designing Cala Maria, The character went through a notable amount of changes before settling with the final design.
Before being settled for having a octopus on her head. Her face originally had fish fins on the side of her face, she had various different types of crowns(along with different hair styles) a starfish was considered to be used at one point, she had a round earring on her nose, a crab tattoo on her right arm, and wearing gloves.
It was also considered for her to hold a weapon(A sword and an anchor). As well the following for her attacks. Cala using her tail, summoning swordfish, a giant crab, and using a title wave.
Before being settled for having a octopus on her head. Her face originally had fish fins on the side of her face, she had various different types of crowns(along with different hair styles) a starfish was considered to be used at one point, she had a round earring on her nose, a crab tattoo on her right arm, and wearing gloves.
It was also considered for her to hold a weapon(A sword and an anchor). As well the following for her attacks. Cala using her tail, summoning swordfish, a giant crab, and using a title wave.

In the games files, you can find an unused boss based off of a Pachinko machine that goes by the name "Pachi-Pachi".
The developers planned to surpass the Guinness World Record for number of boss battles in a run and gun game by having over 30 to the record's 25.