Poison/Dark-Type Pokémon Are Only Weak To Ground
Poison/Dark-type Pokémon are weak to Ground-type moves alone. They also take half damage from Dark, Grass, Ghost, and Poison-type moves and are totally immune to Psychic-type moves. These resistances make them valuable in many situations. However, Ground-type moves are rare, with only 31 (3.32% of all moves) existing as of Generation 9. Despite this, there are seven Pokémon with this type combination, spread over four evolutionary lines:
Hisuian Qwilfish and Overqwil, introduced in Pokémon Legends: Arceus, are technically Dark/Poison-types, but functionally identical to Poison/Dark-type. This makes them a small fraction of all Pokémon but easier to remember.
Water/Ground-Type Pokémon Are Only Weak To Grass
Water and Ground-type Pokémon are individually weak to Grass, so Water/Ground Pokémon take 4x damage from Grass-type moves. On the other hand, they take half damage from Poison, Rock, Steel, and Fire-type moves. They are also immune to Electric-type moves thanks to being part Ground-type. This type combination comes from five different evolutionary families, including:
Since Mega Pokémon don't count as separate Pokémon and Gastrodon has two forms, there are a total of nine Water/Ground Pokémon. This makes them the most prolific of all Dual-type Pokémon with a single weakness. The last time such a Pokémon was introduced was in Omega Ruby and Alpha Sapphire, 10 years ago.
Bug/Steel-Type Pokémon Are Only Weak To Fire
Bug/Steel-type Pokémon have a 4x weakness to Fire-type moves. They also take half damage from Normal, Bug, Steel, Psychic, Ice, Dragon, and Fairy, and only quarter damage from Grass-type moves. They are also completely immune to Poison-type moves. There are only six total Pokémon with this type combination:
Genesect is the only Mythical Pokémon with a single weakness, arguably making it the rarest of all such Pokémon. It even has better resistances than any Legendary Pokémon. Given the versatility of its signature Techno Drive move, Genesect can be the answer to many different opposing Pokémon.
Ghost/Dark-Type Pokémon Are Only Weak To Fairy
Ghost/Dark-type Pokémon once had no Weaknesses at all. This changed with the introduction of Fairy-type Pokémon in Pokémon X and Y, giving them their only weakness. They still take half damage from Poison and are immune to Normal, Fighting, and Psychic-type moves. Despite their utility, there are only two Pokémon with this type combination, making them tied for being the rarest of all Dual-type Pokémon combinations with one weakness:
Spiritomb, though not the first Ghost/Dark-type, is the more recognizable of the two. Its strength was evident upon its debut in Gen 4, with one even being a part of Sinnoh Champion Cynthia's team. Spritomb's strength mainly lies in its defensive potential rather than its attacks. However, with the introduction of Fairy-type Pokémon, Spiritomb is definitely no longer as much of an obstacle in battles as it once was.
Normal/Ghost-Type Pokémon Are Only Weak To Dark
Pokémon Legends: Arceus introduced the first Normal/Ghost-type Pokémon with Hisuian Zorua and Hisuian Zoruark. These Pokémon are only weak to Dark-type moves, which is a pretty good trade-off, as they are immune to Normal, Fighting, and Ghost-type moves. In addition to this, they take half the normal amount of damage from Poison and Bug-type moves.
Normal and Ghost-types are complete opposites, with both being naturally immune to the other. This makes this combination particularly interesting, as well as the fact that it is confined to a single evolutionary family. Uniquely, Hisuian Zorua and Zoroark are immune to both of their own types, something which not is true of any other current Pokémon.
