Funny thing, originally Dry Bowser is actually just Bowser, but dead (undead?).
He became like that when thrown into lava in New Super Mario Bros world 1, came back fighting as a skeleton in the last world, then Bowser Jr threw his bones into black magic soup or something and that revived him as good old meat Bowser.
But after that Dry Bowser became a separate character, probably because Nintendo loves having dozens of slightly different variations for every character.
From now on, I will always think of regular Bowser as “Meat Bowser”.
Just saying, Paper Mario and Pink Gold Peach are referred to by material too, it makes sense. Also there’s a gorilla that’s entirely made of Funk, which as everyone knows is one of the five classical elements of the universe.