I didn't count spin-offs, I will if you want though. :P
Red/Blue
Route 2: Abra for Mr. Mime.
Underground Path (Rt. 5-6): Nidoran♂ for Nidoran♀.
Route 11: Nidorino for Nidorina.
Route 18: Slowbro for Lickitung.
Cerulean City: Poliwhirl for Jynx.
Vermilion City: Spearow for Farfetch'd.
Pokémon Lab: Raichu for Electrode. Venonat for Tangela. Ponyta for Seel.
Yellow
Route 2: Clefairy for Mr. Mime.
Underground Path (Rt. 5-6): Cubone for Machoke.
Route 11: Lickitung for Dugtrio.
Route 18: Tangela for Parasect.
Pokémon Lab: Golduck for Rhydon. Growlithe for Dewgong. Kangaskhan for Muk.
Gold/Silver
Violet City: Bellsprout for Onix.
Goldenrod Department Store: Drowzee for Machop.
Olivine City: Krabby for Voltorb.
Blackthorn City: Dragonair for Rhydon.
Route 14: Chansey for Aerodactyl.
Pewter City: Gloom for Rapidash.
Crystal
Violet City: Bellsprout for Onix.
Goldenrod Department Store: Abra for Machop.
Olivine City: Krabby for Voltorb.
Blackthorn City: Dragonair for Dodrio.
Route 14: Chansey for Aerodactyl.
Pewter City: Haunter for Xatu.
Power Plant: Dugtrio for Magneton.
Ruby/Sapphire
Rustboro City: Slakoth for Makuhita.
Fortree City: Pikachu for Skitty.
Pacifidlog Town: Bellossom for Corsola.
FireRed/LeafGreen
Route 2: Abra for Mr. Mime
Underground Path (Rt. 5-6): Nidoran♂ (FR)/Nidoran♀ (LG) for Nidoran♀ (FR)/Nidoran♂ (LG).
Route 11: Nidorino (FR)/Nidorina (LG) for Nidorina (FR)/Nidorino (LG).
Route 18: Golduck (FR)/Slowbro (LG) for Lickitung.
Cerulean City: Poliwhirl for Jynx.
Vermilion City: Spearow for Farfetch'd.
Pokémon Lab: Raichu for Electrode. Venonat for Tangela. Ponyta for Seel.
Emerald
Rustboro City: Ralts for Seedot.
Fortree City: Volbeat for Plusle.
Pacifidlog Town: Bagon for Horsea.
Battle Frontier: Skitty for Meowth.
Diamond/Pearl/Platinum
Oreburgh City: Machop for Abra.
Eterna Condominiums: Buizel for Chatot.
Snowpoint City: Medicham for Haunter.
Route 226: Finneon for Magikarp.
HeartGold/SoulSilver
Violet City: Bellsprout for Onix.
Goldenrod Department Store: Drowzee for Machop.
Olivine City: Krabby for Voltorb.
Blackthorn City: Dragonair for Dodrio.
Route 14: Chansey for Aerodactyl.
Pewter City: Haunter for Xatu.
Power Plant: Dugtrio for Magneton.
Saffron City: Pikachu for Pikachu.
Silph Co.: Forretress for Beldum.
Diglett's Cave: Bonsly for Rhyhorn.
Olivine Gym: (Any Pokémon) for Steelix.
Black/White
Nacrene City: Cottonee (B)/Petilil (W) for Petili (B)/Cottonee (W).
Driftveil City: Minccino for Red-Striped Basculin (B)/Blue-Striped Basculin (W.)
Route 7: Boldore for Emolga.
Route 15: Ditto for Rotom.
Undella Town (summer): Cinccino for Munchlax.
Black 2/White 2
Route 4: Cottonee (B2)/Petilil (W2) for Petili (B2)/Cottonee (W2).
Route 7: Emolga for Gigalith.
Humilau City: Mantine for Tangrowth.
Route 15: Ditto for Rotom.
Accumula Town: Hippowdon for Alakazam.
Yancy/Curtis Daily HA Pokémon Trades
X/Y
Santalune City: Bunnelby for Farfetch'd.
Cyllage City: Luvdisc for Steelix.
Vaniville Town (Any Pokémon) for Chespin (Chester)/Fennekin.(Kinniekins)/Froakie (Froabble).
Lumiose City: (Any Pokémon) for Ralts.
Kalos hotels: Gyarados for Magikarp. (Any Pokémon) for Eevee.
Omega Ruby/Alpha Sapphire
Rustboro City: Slakoth for Makuhita.
Fortree City: Spinda for Skitty.
Pacifidlog Town: Bellossom for Corsola.
Sun/Moon
Route 2: Spearow for Machop.
Route 5: Lillipup for Bounsweet.
Konikoni City: Zubat for Poliwhirl.
Malie City: Pancham for Happiny.
Tapu Village: Haunter for Alolan Graveler.
Seafolk Village: Granbull for Steenee.
Poni Gauntlet: Bewear for Talonflame.
Ultra Sun/Ultra Moon
Route 2: Spearow for Hawlucha.
Route 5: Lillipup for Noibat.
Royal Avenue: Tentacool for Barboach.
Route 8: Trumbeak for Abrok.
Tapu Village: Phantump for Phantump.
Seafolk Village: Granbull for West Sea Shellos.
Poni Gauntlet: Bewear for Tauros.
Let's Go, Pikachu!/Let's Go, Eevee!
Cerulean City: Rattata for Alolan Rattata.
Vermilion City: Geodude for Alolan Geodude.
Lavender Town: Diglet for Alolan Diglet.
Celadon City: Sandshrew (LGP)/Sandshrew (LGE) for Alolan Sandshrew (LGP)/Alolan Vulpix (LGE).
Saffron City: Raichu for Alolan Raichu.
Fuchsia City: Marowak for Alolan Marowak.
Cinnabar Island: Grimer (LGP)/Meowth (LGE) for Alolan Grimer (LGP)/Alolan Meowth (LGE).
Pokémon League: Exeggutor for Alolan Exeggutor.
Sword/Shield
Motostoke: Bunnelby for Skwovet.
Turffield Stadium: Galarian Meowth for Meowth.
Hulbury: Minccino for Cottonee.
Hammerlocke: Toxel for Togepi.
Stow-on-Side: Maractus for Hatenna (SW)/Impidimp (SH).
Ballonlea Stadium: Galarian Yamask for Yamask.
Circhester: Vanillish for Throh (SW)/Sawk (SH).
Spikemuth: Obstagoon for Mr. Mime.
Wyndon: Frosmoth for Duraludon.
-------------------The Isle of Armor-------------------
Brilliant Diamond/Shining Pearl
Oreburgh City: Machop for Abra.
Eterna Condominiums: Buizel for Chatot.
Snowpoint City: Medicham for Haunter.
Route 226: Finneon for Magikarp.
