Question

What does "???" mean to mean? How do you get the other types, is getting all types possible?

Answer 1

Arceus ??? is a bit of game coding needed for Arceus to change type and can't be obtained legitimately. The ??? type is a special type used only for the move Curse (in B/W it was changed to Ghost).

Arceus can change to other types if it is given a plate of the type you want it to change to as a hold item.

Comments

??? Comes from Curse (Move)'s type by the way. ~

Answer 2

Arceus ??? means that you can change its type and will also be that way. And to change Arceus type is to give it plates