I heard that in Gen IV Arceus could become this type.
Unlike other types, the ??? type is not programmed to have weaknesses or resistances: it is effectively neutral. Due to this, however, issues arise if damaging moves are hacked to be of this type in the Generation II and Generation III games: since it was considered neither a physical nor a special move, the damage dealt by a ???-type move is minimal, even if the move has 255 power, the highest possible.
-Bulbapedia
Fun Fact: Curse, Judgement and Weather Ball(Castform uses it in PokemonXD: Gale of Darkness when the weather changing move Shadow Shadow Sky is used) are the only ???-Type moves.
The ??? type has no weaknesses or strengths, they do normal damage to all types, and ???-type Pokemon receive normal damage from all other types.
My source was my own information and tests when I was playing colleseum and XD. It's also mentioned on Bulbapedia here.