Gen I expansion by game
Gold and Silver (19)
>Crobat, Espeon, Umbreon, Blissey
>Pichu, Cleffa, Igglybuff, Tyrogue/Hitmontop, Smoochum, Magby, Elekid
>Bellossom
>Politoed, Slowking, Scizor, Steelix, Kingdra, Porygon2
Diamond and Pearl (12)
>Mime Jr., Munchlax
>Magnezone, Lickilicky, Tangrowth, Rhyperior, Electivire, Magmortar
>Happiny, Leafeon, Glaceon, Porygon-Z (also considered Gen II expansion)
HeartGold and SoulSilver (1)
>Spiky-eared Pichu
X and Y (14)
>Mega Venusaur, Mega Charizard X, Mega Charizard Y, Mega Blastoise, Mega Alakazam, Mega Gengar, Mega Kangaskhan, Mega Gyarados, Mega Pinsir, Mega Aerodactyl, Mega Mewtwo X, Mega Mewtwo Y
>Mega Scizor (also considered Gen II expansion)
>Sylveon (also considered Gen II and IV expansion)
Omega Ruby and Alpha Sapphire (10)
>Mega Beedrill, Mega Pidgeot
>Mega Slowbro, Mega Steelix, Cosplay Pikachu (Rock Star, Belle, Pop Star, Ph. D, Libre) (also consider Gen II expansion)
Sun and Moon (24)
>Rattata/Raticate, Raichu, Sandshrew/Sandslash, Vulpix/Ninetales, Diglett/Dugtrio, Meowth/Persian, Geodude/Graveler/Golem, Grimer/Muk, Exeggutor, Marowak
>Pikachu (Original Cap, Hoenn Cap, Sinnoh Cap, Unova Cap, Kalos Cap, Alola Cap)
Ultra Sun and Ultra Moon (1)
>Pikachu (Partner Cap)
Let's Go Pikachu and Let's Go Eevee (2)
>Partner Pikachu and Partner Eevee
Sword and Shield (28)
>Pikachu (World Cap)
>Gigantamax (Charizard, Butterfree, Pikachu, Meowth, Machamp, Gengar, Kingler, Lapras, Eevee, Snorlax)
>Meowth/Perrserker, Ponyta/Rapidash, Farfetch'd/Sirfetch'd, Weezing, Mr. Mime/Mr. Rime
>Updates/DLC (Venusaur, Blastoise, Slowpoke, Slowbro, Slowking, Articuno, Zapdos, Moltres)
Legends Arceus (5)
>Growlithe/Arcanine, Voltorb/Electrode
>Kleavor
Scarlet and Violet (9)
>Tauros (Combat/Blaze/Aqua Breed)
>Wiglett/Wugtrio, Toedscool/Toedscruel
>Scream Tail, Sandy Shocks