Accually, ??? was introduced in Gen.2.
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If I may bring mind from Bulbapedia: "??? type Arceus's sprite was made due to hackers being able to change a Pokémon's type, while Arceus's look changes upon type. Thus, they could change Arceus to ??? type, but the game would chrash due to no sprite." It's not exactly from there, but that sums it up.