One Of A Kind
This Pokémon is the only Bug/Ghost type.
Shedinja
#25 is famously known as Pokémon's mascot, being shown front and centre in most Pokémon announcements, along with being one of the few Non-Fully Evolved Pokémon to have a Gigantamax Form in Sword and Shield alongside Eevee, Meowth, and (at the time) Duraludon.
Pikachu
This Champion-ranked trainer is famously known for her Garchomp. She has made cameos in many future games as challenging trainers to fight in the post game, and has two ancestors in Legends Arceus, one as the post game's antagonist.
Cynthia
Famously known for its amazing stats and imposing design, this Pokémon was ruined by the ability Slow Start
Regigigas
This Pikachu "clone" represents Breeding.
Pichu
This Pokémon is the only Rock/Dark type.
Tyranitar
#112 was one of the first Pokémon ever created, further backing up this information is that it has an index number of 001 in the first generation of Pokémon games.
Rhydon
This character is the player character in the generation 1 games, and serves as a post game final challenge in Heart Gold and Soul Silver on Mt. Silver.
Red
Tied with Regigigas for having the highest Attack stat of all normal Pokémon, this Pokémon was ruined by it having Truant as its ability.
Slaking
The Pikachu clone for this region represents the Fairy type.
Dedenne
This is the only Pokémon with the ability Curious Medicine.
Galarian Slowking
#494 is the only Pokémon to have 0 as its regional dex number of its debut region. It is also the only Mythical in its debut generation to only have one form.
Victini
This trainer belongs to Blueberry Academy's Elite Four, specializing in the Fairy typing. She is also canonically Clay's daughter, and uses an Excadrill despite not being a Fairy-type.
Lacey
This Pokémon lost all defensive and supportive utility upon evolving, as it lost Regenerator pre-mega and gained Healer instead, making it an objective downgrade.
Audino
These Pokémon are said to represent Double Battles (the newest feature at the time), as they are meant to be used together.
Plusle & Minun
This Pokémon is the only Non-Legendary with Slow Start.
Varoom
#567 is the only Pokémon whos base stat total matches its National Dex number. It's Pokédex number also references the Dewey Decimal number for feathered dinosaurs at 567.9.
Archeops
This trainer plays the main role in Omega Ruby and Alpha Sapphire's post game. She is known for being a Lore Keeper and a Dragon-type specialist. She also has a Whismur companion named Aster.
Zinnia
This Pokémon's Mega Evolution does not retain the newly added Sharpness ability given to it, as Mega Evolution is not currently present.
Gallade
The Pikachu clone for this region represents temporary form changes, which loosely tie with the regional gimmick.
Morpeko
This is the only Pokémon with Pressure as its secondary ability.
Aerodactyl
#357 is an often forgotten Pokémon. Out of all the Non-Legendary Pokémon available in the DPPt line of games, it is the only one to be exclusive to Platinum by wild encounter. It was also the first Pokémon to ever be distributed in a Cherish Ball way back in 2007.
Tropius
Despite never battling the player, this trainer is shown to have a Croagunk. He also shows up in of a handful of future Pokémon games, having a major role in both XY and USUM's post game.
Looker
This Pokémon was intentionally made so it could never get a speed buff from its ability at any useable level. The only time it can get that speed buff is at level 9 or less.
Brute Bonnet
This Pikachu clone is the only Pokémon with Motor Drive as its hidden ability.
Emolga