Dual-type combinations in Pokémon can be beneficial or detrimental to a Pocket Monster's battling capabilities. With the right combination, a Pokémon can have few weaknesses and a large number of resistances. Even though a large portion of the creatures in the franchise has two types, certain type combinations are only present on a handful of Pocket Monsters.
One example is the type combination of Normal and Ghost, which is currently not available in the series. However, Legends: Arceus will introduce Hisuian forms for Zorua and Zoroark that will have this dual-type combination. Some Pokémon with unique dual-typing are only available in a few games, have a limited number of encounters, or are Mythical, making these combinations even rarer than others.
Fighting & Ghost
Belonging solely to the Generation VII Mythical Marshadow, the dual-type combination of Fighting and Ghost has its strengths and weaknesses. When defending, it helps its Pokémon a fair amount by giving it four weaknesses, three resistances, and two immunities. The immunities are helpful, but only a small number of resistances leaves it open to neutral damage from most of the series' types.
Offensively, the type combination is incredible. On their own, the Ghost-type cannot hit the Normal-type, and Fighting cannot hit Ghost. However, Ghost is super-effective against itself and Fighting is super-effective against Normal. In addition, Ghost can take care of Psychic, which resists Fighting, and Fighting can take care of Dark, which resists Ghost. The two types complement each other very well offensively.
Fire & Water
One thing that makes Volcanion one of the coolest Fire-type Pokémon is its combination of two of the most popular types in the series: Fire and Water. The joining of these two types is more than decent defensively because it is only weak to three types and resists five others.
When it comes to offense, the combination of Fire and Water can also be viable. Together, they are super-effective against seven types, and the best part is that Fire can deal with Grass-types, one of Water's weaknesses, while Water can deal with Rock and Ground-types, two of Fire's weaknesses.
Ice & Fire
The combination of Ice and Fire is only accessible on Galarian Darmanitan when it enters Zen Mode, which only occurs if the Pokémon has Zen Mode as its Ability and reaches half of its maximum health. The type combination is decently capable on defense, being vulnerable to four types and resisting four others. It has an extreme vulnerability to Rock, though, which can present issues.
On the attacking side, it deals super-effective damage to seven types, one of which is also one of the type combination's weaknesses in Ground. The two types complement each other against certain types, making it a viable, though not exceptional, offensive combination.
Fighting & Ice
Crabominable is one of the most underrated Pokémon from the Alola region, but its unique typing of Fighting/Ice makes it more interesting. To obtain it, trainers must evolve a Crabrawler on Mount Lanakila, which, while not terribly difficult, makes it a more challenging dual-type combination to acquire than most. Unfortunately, the type combination is not one of the best defensively.
It is weak to a third of the types in the franchise while only resisting three. Offensively, it is more than decent, though. Combined, they are super-effective against nine of the series' types. As such, the combination of Fighting and Ice is much better offensively than it is defensively, where it does not excel.
Rock & Poison
The Ultra Beast Nihilego is currently the only Pokémon to have a type combination of Rock and Poison. Defensively, this combination offers some benefits like resistance to a third of the types in the series. However, it also offers four weaknesses, which is not terrible but not exceptional either, especially because it has an extreme vulnerability to Ground.
On the offensive side, the typings work decently, but not incredibly, well together because neither greatly makes up for the other's offensive weaknesses, aside from doing some neutral damage. They are also both walled by Steel and, to a lesser extent, Ground.
Dragon & Ice
Kyurem is one of the Legendary Pokémon with the best lore, so it is fitting that the typing shared by its three forms is one of the more fascinating. Offensively, it is only decent. Dragon-type Pokémon tend to be some of the strongest, but based solely on its type capabilities, Dragon is only super-effective against itself. However, both the Dragon and Ice-types can deal with other Dragon Pokémon that exist in the series. In addition, Ice is super-effective against three types aside from Dragon.
Defensively, it has five type weaknesses and only resists three types. As such, the combination is decent but not fantastic, though it makes for an interesting dual-typing for a Legendary Pokémon like Kyurem.
Dragon & Fairy
Only accessible through Mega Altaria, which itself is only obtainable in a small handful of games, the combination of Dragon and Fairy is one of the rarest around. Though many powerful Pokémon make up either of these two types, the combination is a bit of a mixed bag when it comes to type-based advantages and disadvantages.
The Fairy typing allows immunity to Dragon-type attacks, one of the Dragon-type's biggest weaknesses, and the combination has four other type weaknesses. However, it has seven type resistances, which helps it significantly. Offensively, Dragon does not add anything to the equation to assist Fairy except for neutral damage to Poison and Fire, which resist Fairy.
Fire & Steel
The dual-type combination of Fire and Steel is only present on the Generation IV Legendary Heatran, and it is generally fantastic on the defensive side. Though its extreme vulnerability to Ground-type attacks can be concerning, it only has two other weaknesses. In addition, it resists nine types and is immune to Poison.
Offensively, it is not as impressive, but it is still viable. Fire is super-effective against Steel, which resists itself, and Steel is super-effective against Rock, which resists Fire. The combination of Fire and Steel is better on defense, but it is all around relatively great.
Poison & Fairy
Though it is prevalent in several locations in Pokémon: Sword & Shield, thanks to Galarian Weezing, who utilizes both types in its aesthetics to be one of the best-designed regional forms, the combination of Poison and Fairy is nowhere else to be found in the rest of the series. Defensively, the type combination is relatively capable due to only being weak to three types while resisting five types.
It has the additional benefit of being immune to Dragon-type attacks. The combination is only decent offensively. While the Poison-type is less viable, it contributes by offering super-effective attacks against Fairy, which has earned a reputation as one of the strongest types in the franchise because of the Pokémon that have it.
Rock & Fighting
With Terrakion as its sole Pokémon, the dual-type combination of Rock and Fighting has some serious issues on the defensive side. It is weak to seven types in the franchise, but it also resists six. The Flying-type, which is super-effective against Fighting, does neutral damage against the combination, but aside from this, the typings do not defensively help each other much at all.
Offensively, however, it is a different story. The two are super-effective against nine of the Pokémon types, complementing each other in several ways and allowing the Pocket Monster that has the combination to be a dominant force.