The ??? type does not exist in Generation I. It exists in Generations II, III, and IV solely to be the type of the status move Curse; in Generation III, Eggs are also marked as ???-type. Unlike other types, the ??? type does not have any weaknesses or resistances.
If damaging moves are hacked to be of this type in the Generation II or III games, since it is considered neither a physical nor a special move, the damage dealt by a ???-type move is minimal, even if the move had the highest possible base power.
In the games
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 completely 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, to maintain the fact that the identity of the Pokémon inside is supposed to be unknown. This does not appear in Generation II, which uses a special status screen for Pokémon Eggs different from that of normal 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 considered as Normal-type Pokémon by the game.
Weather Ball's type changes depending on the weather condition. If Weather Ball is used when the field is under the effect of Shadow Sky, Weather Ball changes to a ???-type move and will be neither super effective nor not very effective against any type. Despite this, if a Pokémon with Color Change is hit with this move under these conditions, it will be changed to a Normal-type Pokémon.
This situation is the only way to legitimately encounter the type effectiveness of a ???-type move.
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 Pokémon Diamond, Pearl, Platinum, HeartGold, and SoulSilver; however, it is never otherwise used without hacking.
In Generation IV, if a pure Flying-type Pokémon uses Roost, it becomes typeless until the end of the turn, effectively behaving as if it were ???-type. However, in Generation IV, all Flying-type Pokémon have two types (except Arceus holding the Sky Plate, who is unable to learn Roost). To obtain a ???-type Pokémon, the player must change a Pokémon to be pure Flying-type (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 until the end of the turn instead.
The game will not allow the player to use Conversion to become ???-type. Curse, the only move of this type, will be ignored by Conversion, such that if a Pokémon uses Conversion while the only other move it knows is Curse, Conversion will fail.
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.
The ??? type is not present in the coding of the Generation V games, with the displayed typing of Eggs having already been removed the previous generation and the type of Curse being newly changed to Ghost. This was possibly because it necessitated making many special cases that would otherwise be unnecessary. Since the only official use of the type at the time of its retirement was for a single non-damaging move, the type itself was essentially useless.
- This is the only type in the core series games to not have any Pokémon with it as a type.