Ever noticed how in every new generation, that region's PokeDex ALWAYS starts off with three Grass-type Pokemon, then three Fire-type Pokemon, and then three more Water-type Pokemon? JEEEZ-zus Game Freak! Stop being so unoriginal!!

But actually this kind of common thread across each generation has been something that has always fascinated me. In fact, how each generation implements these common threads AND to what degree is the kind of stuff which makes it "Pokemon" in my eyes.

So with this said, here's my list of common Pokemon threads and which Pokemon from what generation fits the bill. Of course your mileage may vary... but if you spotted a thread that I missed, or maybe think another Pokemon fits in better, then I'm all ears!

 

THREAD G1 – RGBY G2 – GSC G3 – RSE G4 – DPP G5 – BW G6 – XY G7 – SM G8 – SwSh
Grass Starter Bulbasaur Chikorita Treecko Turtwig Snivy Chespin Rowlet Grookey
Fire Starter Charmander Cyndaquil Torchic Chimchar Tepig Fennekin Litten Scorbunny
Water Starter Squirtle Totodile Mudkip Piplup Oshawott Froakie Popplio Sobble
First Bugs Caterpie, Weedle Ledyba, Spinarak Wurmple Kricketot Sewaddle, Venepede Scatterbug Grubbin Blipbug
First Birds Pidgey Hoothoot Tailow Starly Pidove Fletchling Pikipek Rookidee
First Rodent Rattata Sentret Zigzagoon Bidoof Patrat Bunnelby Yungoos Skwovet
Pikachu Clone Pikachu Marill Plusle, Minun Pachirisu Emolga Dedenne Togedemaru, Mimikyu Morpeko
Annoying Zerg Rush Zubat Unown (?) Wingull Woobat
Rocker line Geodude line Larvitar line Aron line Rogenrola Biancle Rolycoly
Psychic line Abra line Ralts line Bronzor Gothita, Solosis Espurr Hatenna
Muscle line Machop line Meditite, Makuhita Riolu Timburr Pancham Stufful
Steel line Beldum Klink Honedge
Ew Gross Grimer, Koffing Gulpin Trubbish
Eeveelution Eevee, Vaporeon, Jolteon, Flareon Espeon, Umbreon Leafeon, Glaceon Sylveon
Fossils Omanyte, Kabuto, Aerodactyl Lileep, Anorith Cranidos, Shieldon Tirtouga, Archen Tyrunt, Amaura Dracozolt, Dracovish
Pseudo-legendary Snorlax Sudowoodo Kecleon, Castform Rotom Zoura
Legendary Blue Articuno Suicune Regice Azelf Cobalion, Thundurus Tapu Fini
Legendary Yellow Zapdos Raikou Regirock Uxie Virizion, Tornadus Tapu Koko
Legendary Red Moltres Entei Registeel Mesprit Terrakion, Landorus Tapu Lele, Tapu Bulu
Big Dragon Dratini line Bagon line Gible Axew, Deino Goomy Jangmo-o Dreepy
Story Legends Lugia, Ho-Oh Kyogre, Groudon, Rayquaza, Latias, Latios Dialga, Palkia, Giratina Reshiram, Zekrom, Kyurem Xerneas, Yveltal, Zygarde Solgaleo, Lunala Zacian, Zamazenta
Final Powerful Mewtwo Deoxys Cresselia, Darkrai, Arceus Genesect Volcanion (Ultra Beasts) Eternatus
Final Mystery Mew Celebi Jirachi Phione, Shaymin Victini, Keldeo, Meloetta Diancie, Hoopa Magearna, Marshadow
Fishy
Furry Bait
Fire Mammal
Weird One-Off

OK, that's about it for now... I think there's more than a few threads I missed, like all the general fishies, but also Old Rod crap catches, etc... but this is a pretty good start for now.