Ghost
Effect
Ghosts are rare Pokémon, and the only type to have two immunities. In total there are just 34 Ghost type Pokémon (not including Megas/Formes), slightly above Ice.
In the first generation, Ghost moves has no effect on Psychic Pokémon, however, it was later changed to be super-effective. When paired with the Dark type it was the only type combination to have no weaknesses prior to Gen 6.