Version-exclusive Pokémon in Let's Go Pikachu & Let's Go Eevee
Like all the paired games in the Pokémon series, Let's Go Pikachu & Let's Go Eevee each include Pokémon that cannot be obtained in the alternate version. To obtain Pokémon missing from your version you need to trade with someone who has the opposite version.
The known version exclusives so far are the same as those in the original Pokémon Red & Pokémon Blue, though they are spread differently. Therefore, it's possible the following Pokémon may also be found only in one of the versions Let's Go Pikachu or Let's Go Eevee: Ekans/Arbok, Mankey/Primeape, Electabuzz, Magmar, Scyther and Pinsir.