Pokemon filling these unused type combinations that I've designed for my fictional game outline:
Fire/Grass - Sunflower Pokemon that makes heavy use of Sunny Day and various move that care about sun (eg. Synthesis).
Poison/Steel - Pokemon representing pollution and over-industrialization; think Hexxus from Fern Gully. (I actually named one of its abilities "Toxic Love." 😁) Very powerful (it's actually the game's "second" psudo-legendary) and was used as a weapon of war by an evil army in the backstory.
Ground/Fairy - Badger Pokemon that plays on the actual meaning of the Irish phrase
aos si (from which the words sith/sithi/sidhe are derived) as "people of the mounds." Ace of the Fairy type Gym Leader due to Ground being strong against both of Fairy's weaknesses, and as such I made a point of making this Pokemon deliberately awesome.
Bug/Dragon - A dragonfly, something which should have been done properly a long time ago and which Flygon epically fails at. Starts as Bug/Water, then becomes Bug/Steel, then Bug/Dragon. The game's "primary" psudo-legendary.
Normal/Bug and Normal/Rock - A pair of version exclusive Pokemon based on the Beatles and the Rolling Stones. 😁 (They're Normal type to get STAB for sound-based moves.)
Bug/Dark - Female-only counterpart to Scyther; comes with a new baby Pokemon that can evolve into either. The poster child for a new Dark type move called Heartbreak that deals double damage if the target is infatuated with the user.
Fighting/Ground - A Pokemon that I haven't fleshed out that much yet in terms of what it
is, but is the counterpart to a (also not very developed) Grass/Ground type. The latter has a signature ability that causes it to automatically plant its roots upon entering battle as though it had used Ingrain; while the former has a signature move (possibly called Uproot if that's not too on the nose) that is similar to Dragon Tail and Circle Throw but with the added property of being able to bypass Ingrain and Suction Cups. The idea is that the former Pokemon is one of the few natural predators of the latter due to being one of the few things capable of pulling it up.
Rock/Ghost - A three-stage-evolution Pokemon that is a bunch of possessed pieces of stone statue assembling themselves into a full body. Has a signature ability that allows it to effectively have multiple abilities simultaneously depending on how you evolve it, that functions similarly to Calyrex's ability As One.
Fighting/Fairy and Dragon/Fairy - A pair of version-exclusive Pokemon that are the male and female versions (respectively) of the same species, like Tauros and Miltank or the Nidorans. Both epic warriors that defend their home and friends from invaders.
This and
this are rough approximations what they look like, or at least of the vibe I'm going for. The "third" Legendary of the games (equivalent of Rayquaza or Giratina) is also a Dragon/Fairy type; I haven't developed this one much, but I envision it as being similar to Ugin from Magic The Gathering.
A few more type combinations that have technically been done before, but where like Mega Altaria I kind of feel like they "don't count":
Psychic/Electric (used for Alolan Raichu, which I think is barely a Psychic type) - A Pokemon that channels "life energy" and "auras" in the eastern sense, but also channels "physical" forces like gravity and electromagnetism, with the idea that all of these things are just different forms of energy that are all equally alike. I like to describe this Pokemon as being a cross between a yogi and Magneto. It's name, of course, involves a pun on the word "om/ohm." This is quite possibly my favorite out of all of the Pokemon I've designed. 😄
Fire/Water (only used on the mythic Volcanion, and is far too interesting a type combination to be relegated to a mythic) - A Fire type lizard that eats snow for nourishment, "fuel" (think Coalossal), and regurgitate-attack fodder, and evolves into either a Fire/Rock type or a Fire/Water type depending on what moves it knows when it evolves. The Fire/Water type is a crocodilian-like lizard that shoots steam out of its limbs and the end of its tail to go very fast.
Fire/Ice (only seen on Galarian Darmanitan Zen Mode, which is a very cool Pokemon but is difficult to obtain due to needing to have its hidden ability) - I have two of these! One is a humaniod Pokemon that channels and manipulates heat energy; for example, it has an Ice type attack called Thermodrain that boosts the power of the next move used if that move is Fire type. The other is the evolution of a regional Onix, and is inspired by the Ice Worms from Subnautica Below Zero.
Electric/Fire (only seen on one of Rotom's forms) - The game's Pikachu clone, that is based on the idea of a "short circuit" or "blown fuse"; an electrical malfunction that sets stuff on fire. Its signature ability, which has to do with boosting Evasion, is called Magic Smoke, a pun which I am very proud of. 😁😁 (And no, I did not start with the name; I actually designed the ability before coming up with the name.)
Rock/Fighting (only seen on one of the Swords of Justice) - A geode Pokemon whose middle stage is called "Graplode" and resembles a Geonid from Dungeons and Dragons, and whose third stage involves multiple references to Dwayne Johnson. 😁😉
Water/Fighting (also only seen on one of the Swords of Justice) - A Pokemon based on a mantis shrimp, which the #1 animal that I am flabbergasted has not been made into a Pokemon already.
Fairy/Steel (used on Zacian, but I don't care I want to mention this one 😛) - A regional variant of Pawniard/Bisharp referred to as "White Pawniard / White Bisharp" as opposed to "Alolan Bisharp," representing the other side of the chess board from the original variant.
