The ??? type does not exist in the coding of Generation I. It exists only in Generations II, III, and IV solely to be the type of Eggs and the status move Curse. Unlike other types, the ??? type is not programmed to have weaknesses or resistances.
Issues arise if damaging moves are hacked to be of this type in the Generation II or III games. Since it was considered neither a physical nor a special move, the damage dealt by a ???-type move was minimal, even if the move had the highest possible base power.
The move Curse was given the ??? type when introduced in Generation II, likely because of its mysterious nature: it is so far the only move which differs in its use between Pokémon of different types. It was retyped as Ghost-type move in Generation V. It is a Normal-type move in the Pokémon Mystery Dungeon series.
Pokémon Eggs are displayed as being ???-type in Generation III on their status screen; this is likely because, like all other data, information about a Pokémon Egg is supposed to be sparse, including its original Trainer and ID number. This information does not appear in Generation II, which uses a special status screen for Pokémon Eggs that is different from that of Pokémon, and was removed in Generation IV, when the section of a Pokémon's status screen that displays type was separated from where it was met. From Generation IV on, Eggs are counted as Normal-type Pokémon by the game.
Arceus has the Ability Multitype, which changes its type and appearance depending on the Plate it is holding. Likely as a precaution for Arceus hacked to be ???-type, a ???-type sprite of Arceus is present in Diamond, Pearl, Platinum, HeartGold, and SoulSilver Versions; however, it is never used without hacking. It is also likely that this ???-type Arceus was made so that Arceus wouldn't crash the game whenever it tried using Multitype to change its type, though this is probably not the case due to the lack of a ???-type Kecleon.
In Generation IV, if a pure Flying-type Pokémon uses Roost, it becomes a ???-type Pokémon until the end of the turn. However, in Generation IV, all Flying-type Pokémon have two types (except Arceus, which is unable to learn Roost). To obtain a ???-type Pokémon, the player must change a Pokémon to be a pure Flying-type Pokémon (with Color Change, Conversion or Conversion 2), followed by using Roost. In Generation V, if a pure Flying-type Pokémon uses Roost, it becomes a pure Normal-type Pokémon.
The game will not allow the player to use Conversion to convert to the ??? type. Curse, the only move of this type, will be ignored by Conversion, so that a Pokémon with only Curse and Conversion will result in Conversion failing.
On official material, the ??? type is often used to denote a type that has not yet been revealed. Leading up to the release of Pokémon X and Y, Sylveon's type was listed as ??? on official material until its actual type was revealed to be Fairy-type.
The ??? type is not present in the coding of the Generation V games. This was possibly because making exceptions for a type that is used solely by a single, non-damaging move in the case that hackers would alter a Pokémon or move and crash their game had become tiresome. Considering that the only other "official" uses of the ??? type had been the unavailable form of Arceus and the typing of Eggs in Generation III, the type itself was essentially useless.
- It is the only type to not have any Pokémon with it as a type.
- It is the only type to have been retired.