Just out of curiosity, I wonder if Pokemon is influenced by other anime/manga/game ... stuff like that.
For example, Crocodile from One-piece existed before Gen5 came out - and anyone who's seen him would know he can control sand, which we could also take as "ground". He's also one of the bad guys, which is pretty "dark".
And then there's the Sandile family in Gen5. Ground/Dark.