??? theoretically took and dealt 1× damage with every type, but there were no ??? Pokémon or damaging moves.
In Generation IV, pure Flying-types become ???-type during Roost. This can be accomplished with Conversion, Conversion 2, or the ability Color Change. A Pokémon can legitimately have both Roost and one of these only through the use of Mimic, Sketch, Role Play, or Skill Swap. (No Pokémon that has Trace or Multitype learns Roost, and Multitype cannot be copied.)
Generation IV has sprites for a ???-type Arceus, even though Arceus cannot become ???-type through regular play. Eggs are purely ???-type before hatching before Generation V, and are displayed as such in the Generation III status screen. In Generation V, the ??? type no longer exists.
Ghosts pay half their max HP to hurt the target every turn. Others decrease Speed but raise Attack and Defense.