Which Switch games are the only way to obtain certain Pokemon?

Question

I'm making a living National Dex to hopefully one day substitute in a living Shinydex, and I was thinking about which Pokemon I'd have to ask for on the GTS (in my case the BDSP exclusives) and which I can catch in the games I have.
Any non-timed encounter or gift counts.
If it is a gift, please say how you obtain/activate said gift.

Pokemon like Melmetal (before ZA Mega Dimension) should be mentioned that they cannot be found in a Switch game, but do say where they can be gotten (e.g GO, HOME gift, previous game)

Comments

Edited for a little more clarity, even if you now have to reread the title a few times to get it

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Cant melmetal be found in let’s go games?

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No, you have to transfer it in from Go

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i know paradox pokemon, ogerpon, terapogos, pecharunt, the loyal three for S/V

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And then there's some stuff like the Ledyba line, Castform and Spinda (plus others) all being BDSP exclusive

Answer 1

Legends: Arceus alone has the following lines: Wyrdeer, Ursaluna, Sneasler, and Enamorus.
Note that Stantler and Ursaring cannot evolve in S/V and must evolve them in PLA. (only the Bloodmoon Ursaluna is obtainable in S/V Teal Mask DLC, not regular Ursaluna)

Legends Z-A alone has the following lines: Patrat, Pansage, Pansear, Panpour, Furfrou.

BD/SP alone have the following lines: Ledyba, Taillow, Skitty, Castform, Kecleon, Clamperl, Spinda.
Spinda is a very unique case because while it's only available in BD/SP with the National Dex, due to a glitch you can't transfer it from BD/SP to HOME or vice versa.

Let's Go Pikachu/Eevee does not have any unique species, but it's the only game to have the Alolan Rattata line.

S/V alone has all the Gen 9 Pokemon lines, excluding Annihilape, Capsakid, Tinkatink, Cyclizar, Glimmet, Greavard, Flamigo Dondozo, Tatsugiri, Frigibax, Gimmighoul, Fidough, as they are in ZA DLC.

Credit to budi1100's answer here for everything except SV, I am going to add the SwSh exclusives at some point. Please remind me, though.

Comments

who is budi1100