Every generation, the amazing designers behind Pokémon introduce from a few dozen to more than a hundred new monsters into the series. Each time, new type combinations are introduced that haven't been used before. If these type combos are only attached to one evolutionary family, they're considered "unique type combinations."
Some of these unique combos will be eliminated when another Pokémon with the same typing is introduced in a later generation. However, many retain their unique typing for a long time. Some even have since Generation I. Also, some are definitely more interesting than others. Here are just a few examples of the most interesting ones.
10 Water/Ghost
Imagine heading down to the beach and taking a nice, relaxing dip in the ocean. What a nice, beautiful day, right? Well, now imagine a ghastly jellyfish the size of a small car swimming up to you.
Jellicent is a spooky Pokémon. It's a Ghost/Water-type, and currently, the only one, thank goodness. The last thing the already-terrifying ocean needs is more undead fish floating around in it. It's actually surprising there haven't been more Pokémon with this type combination — it seems like an obvious design inspiration.
9 Ice/Bug
It took until the most recent installment, Generation VIII, to get a combination of Bug and Ice. This may be due to the fact that bugs are not commonly found in icy regions or associated with winter in any way, other than a few rarer species like the Arctic Moth.
Game Freak found a great way to combine these two natural things and take inspiration from those Arctic Moths. Snom and its evolution, Frosmoth, are icy moths that have the Bug/Ice dual typing. The result is surprisingly beautiful.
8 Rock/Poison
This combination, like Ice and Bug, really defies assumptions about natural elements being paired together. Rocks and poison (or in many cases in the Pokémon world, venom) are not typically associated with one another anywhere in nature. Perhaps this is why it took an extra-dimensional and extraterrestrial Pokémon to have this typing.
This jellyfish-like Pokémon comes from the Ultra Space dimension, along with the other Ultra Beasts. It's clear that its Poison-typing comes from being a jellyfish, but the Rock-typing is not as obvious. Some fans theorize that it comes from its glass-like body that's specified in some in-game references. In this case, it's similar to Carbink or Gigalith, who are crystal-like in design.
7 Dragon/Fighting
A dragon-like Pokémon would probably be terrifying in real life, as would many Fighting-type Pokémon, which are typically very strong and scary-looking. However, Generation VII introduced a Pokémon that would combine these two things into one giant, terrifying monster.
The Kommo-o line, which is Generation VII's resident pseudo-legendary, features this unique typing. The typing seems like it'll be great competitively. Unfortunately, Kommo-o has a lack of good STAB moves. Regardless, the type combination is very interesting and hasn't been seen elsewhere.
6 Fighting/Flying
As of Generation VI and the introduction of Hawlucha, the Flying-type has been paired with every other possible type. It is only one of two types to have done this, the other being Water. Hawlucha is a Flying/Fighting-type Pokémon — the only one, in fact.
The typing is very interesting and goes perfectly with Hawlucha's design. It's a bird that is also a luchador. It seems to resemble Aztec eagle warriors or other depictions of other bird-like protector gods. It's a fascinating design that really shows what Pokémon can aspire to.
5 Water/Fire
Speaking of Pokémon that filled the final hole of a type being paired with every other type, Volcanion, a mythical Pokémon introduced late in Generation VI, completed Water's combination list. Volcanion is a Fire/Water-type, with Fire being the only type that Water had not yet been paired with.
This is an interesting type combination, as the two are typically depicted as opposites, perhaps being one of the reasons they've never been used outside of Volcanion. It seems Volcanion's design has taken inspiration from geysers and steam, which is an obvious choice for this type combination.
4 Fire/Steel
Heatran is a legendary Pokémon introduced in Generation IV, and it has a unique Fire/Steel typing. Since Steel has the most resistances of any type, and Fire comes in second place in the same category, the combination of the two is an interesting one.
Due to its typing, Heatran resists a whopping nine types and is immune to one. Also due to its typing, it holds the record for the most double-resistances. Heatran will only take one-quarter damage from Bug, Steel, Grass, Ice, and Fairy moves. However, it is double-weak to Ground-type moves.
3 Poison/Flying
Since there are eight generations of Pokémon and close to a thousand different species, it's impressive that there are still some Pokémon from the first generation with unique type-combinations. What's even more interesting is that all three (the Zubat line, the Gastly line, and Nidoqueen/Nidoking) with unique typing have the Poison-type as one of them.
Zubat is perhaps one of the more interesting ones of the three, as many people consider it a garbage Pokémon that they grow tired of seeing. But, it actually holds status as a unique-type combination, being Poison/Flying. Who knew?
2 Water/Steel
Some unique type combinations are interesting because they're unique when they seem like they could have been used many other times. One example of this is the Water/Steel combination, which really doesn't seem like there would only be one Pokémon for.
But, it is still true. The only Water/Steel Pokémon is Empoleon, one of Generation IV's starters. It doesn't even share the combination with the rest of its evolutionary family, as its pre-evolutions are just pure Water-type. Empoleon's fellow Generation IV starter, Torterra, also has a unique combination, being Grass/Ground — another type that seems like it should have been used more than once.
1 Electric/Poison
One of the most interesting type combinations ever created by Game Freak lends itself to one of the most clever designs in recent memory. Toxtricity is a Poison/Electric-type Pokémon introduced in Generation VIII. It almost seems like the type combination was created before the Pokémon, which was then modeled to fit into the combo.
Toxtricity takes heavy inspiration from guitars and punk rockers in British history (Galar is based on the U.K.). It has a Low Key Form and an Amped Form, based on bass guitars and electric guitars, respectively. It's a fantastic design with clever typing.