There are 171 different type combinations, with several of these belonging to only a single Pokémon. Some dual-type combinations can defy logic, and some come about as a result of some very creative thinking. While it can have its benefits, having rare or unique typing isn't always a good thing.
Many Pokémon gain an additional type when they evolve. This can both help and hinder a Pokémon depending on the weaknesses, immunities, and resistances they lose and acquire. A Pokémon's learnset is also affected by a change in typing, as certain moves can benefit from STAB (Same Type Attack Bonus) when they wouldn't have before.
10 Bibarel Is The Only Normal/Water-Type
When Bidoof evolves into Bibarel, which was introduced in gen 4, it gains Water typing. While it doesn't look much like a Water Pokémon, its category of Beaver Pokémon compared to Bidoof's Plump Mouse category means its habitat is in the water.
While Bibarel gains two weaknesses, Grass and Electric, to go along with Fighting, it also becomes resistant to Steel, Fire, Water, and Ice, thanks to its new typing.
9 Torterra Is The Only Grass/Ground-Type
While it may not read like a very unique dual-type combination, Torterra is the only Pokémon to be both Grass- and Ground-type. The Continent Pokémon is therefore weak to four different types: Flying, Bug, Fire, and Ice, immune to only one: Electric, and resistant to Ground and Rock.
Its learnset is comprised mostly of Grass-type moves, from which it benefits from STAB. Its evolutionary predecessors, Turtwig and Grotle, are both pure Grass-types.
8 Morpeko Is The Only Electric/Dark-Type
Introduced in gen 8, Morpeko has the category of Two-Sided Pokémon, so called because it can appear cute and friendly, but it has a Hangry Mode, when it becomes the opposite. Another duality that Morpeko has is its typing, Electric and Dark.
It boasts resistances to five different types but is also weak to four, and has one immunity, Psychic. Moves such as Aura Wheel, Crunch, and Spark give it STAB.
7 Crabominable Is The Only Fighting/Ice-Type
Crabominable gains Ice typing when it evolves from Crabrawler, which is a pure Fighting-type. The Pokédex entry from Pokémon Sun explains how this came about: "It aimed for the top but got lost and ended up on a snowy mountain. Being forced to endure the cold, this Pokémon evolved and grew fur."
It's one of the least-favorable typings there is, making Crabominable weak to six different types and resistant to three, with no immunities.
6 Galarian Darmanitan Zen Mode Is The Only Ice/Fire-Type
One might struggle to comprehend how a Pokémon can be both Ice- and Fire-type, but Galarian Zen Mode Darmanitan achieves this, and it's perhaps not surprising that it's the only Pokémon that does so. The Standard Form of Darmanitan is a pure Fire-type, but it can change its form during battle depending on its Ability and HP.
Galarian Darmanitan Standard Mode is a pure Ice-Type, and when it changes to Zen Mode it becomes a dual Ice/Fire-type.
5 Marshadow Is The Only Fighting/Ghost-Type
Introduced in gen 7, the Mythical Pokémon Marshadow's Fighting/Ghost typing gives it immunity to Normal and Fighting-type moves and resistances to Poison, Rock, and Bug, but weaknesses to Flying, Ghost, Psychic, and Fairy.
A number of the Gloomdweller Pokémon's moves give it STAB, including Shadow Punch, Close Combat, and Spectral Thief, Marshadow's signature move. Owing to its reticence to appear before humans, Marshadow's existence became the stuff of myth. It learned to fight by mimicking the movements of others from the shadows.
4 Empoleon Is The Only Water/Steel-Type
The other two Pokémon in Empoleon's evolution family, Piplup and Prinplup, are pure Water-types. Empoleon gets its Steel typing from its wings, which are so sharp they can cut through ice floes.
The addition of Steel typing means Empoleon is the only Water-type to not be resistant to Fire-type moves, but it still boasts resistances to ten different types, while also gaining immunity to Poison. Empoleon doesn't have any Steel moves that give it STAB.
3 Mimikyu Is The Only Ghost/Fairy-Type
Introduced in gen 7, what lies beneath Mimikyu's rag was disturbing enough to cause a Trainer to die painfully in the night, according to the Ultra Moon Pokédex entry. While it's not clear where it gets its Fairy typing from, the Disguise Pokémon has an equal amount of Ghost and Fairy moves at its disposal as part of its learnset.
Mimikyu is immune to Normal, Fighting, and Dragon moves, while it is only weak to Ghost and Steel.
2 Volcanion Is The Only Fire/Water-Type
A Pokémon that is both Fire- and Water-type may seem like an oxymoron. Volcanion has this typing because it stores water in its pipes, which it uses to expel billows of steam that are powerful enough to blow away an entire mountain.
Its typing means it is only weak to Ground, Rock, and Electric, and moves like Scald, Flare Blitz, and Steam Eruption, the Steam Pokémon's signature move, give it STAB.
1 Shedinja Is The Only Bug/Ghost-Type
Shedinja not only has a unique dual-type combination, but it also has a unique evolution requirement. If there is space in the player's party and they have a spare Poké Ball when Nincada evolves into Ninjask starting at level 20, Shedinja will appear in their party.
It is essentially an exoskeleton with a halo above its head, which is where it gets its Ghost typing. The Shed Pokémon has a whopping thirteen immunities thanks to its dual-type combination.