3rd Gen Single questionmark MissingNo?

[JoshTheTrainer]

I was wondering something. There's a pretty big amount of ?'s between Celebi and Treecko. I wonder why are there so many questionmark MissingNo.'s between them? Are they all canceled pokemon or something?

 

[Lycanthropy]

I've heard that data of other forms of the Pokémon is stored there, like Castform and Unown.