While it was a long time coming, Cuphead dazzled me with its 1930s cartoon style, a callback not just to the old days of animation, but the individual techniques used by Fleischer and Disney in their earliest works.
It would be Fleischer's movement that influenced the team behind the game. Disney would go through multiple style changes over the years, while Fleischer would maintain a characteristic sense of fluidity to it. And it's obvious that the mermaid/medusa boss was heavily inspired by Betty Boop, one of Fleischer's regular cartoon characters.
Everyone knows how hard the game is, but did you know that the rewards for perfecting all the bosses are even more of an homage to Max Fleischer's style? If you complete all the platforming levels without attacking once, you'll unlock a black-and-white art style that takes the game's visuals further back towards the 1920s.
But I'd argue that the unlockable "2-Strip" style of animation is far more fascinating. This was done by producing an image using two separate cells, green and red, then merging the images together with light to create the illusion of full colour. This 1936 cartoon about a rather dastardly spider is probably the safest example I could find. The game is typically shown in what's known in-game as "3-strip", which is more like Fleischer's later work such as the Superman cartoons.
I'd like to imagine that Disney's recent revival of Mickey Mouse was inspired a little bit by the popularity of Cuphead when the game was first revealed.