From Gen II and up to Gen IV, there was a ??? type. Although no Pokémon had this type, it was the type of the eggs in Gen III, the move Curse was of this type, and Arceus had a ???-type variant programmed into the Gen IV games even though there is no ??? plate, thus no way to access it.
It had no weaknesses or resistance to any type, but since there were no offensive moves or Pokémon of this type, it could not be accessed.
It was removed in Gen V, and Curse became Ghost-type.
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