International Red and Blue are
>Japanese Red and Green's Pokémon availability
>Japanese Blue's translated text, graphics, and layout
Red and Green have many counterpart Pokémon.
>Caterpie/Metapod and Weedle/Kakuna
>Ekans/Arbok and Sandshrew/Sandslash
>Nidoran-F/Nidorina and Nidoran-M/Nidorino
>Vulpix and Growlithe
>Oddish/Gloom and Weepinbell/Bellsprout
>Meowth and Mankey
>Psyduck/Golduck and Slowpoke/Slowbro
>Grimer/Muk and Koffing/Weezing
>Shellder and Staryu
>Krabby/Kingler and Horsea/Seadra
>Scyther and Pinsir
>Electabuzz and Magmar
Not all of them are version-exclusive, but they do have mirrored encounter rates. Caterpie may be less common than Weedle in one area in one game, but the reverse is true in the other. Each counterpart appears in the same locations as each other at the same level between games and may have similarities, like Shellder and Staryu both being Water-types that need a Water Stone to evolve, or Psyduck and Slowpoke both being dopey Water-types with latent psychic powers. The only exception is Electabuzz (Power Plant in Red) and Magmar (Pokémon Mansion in Green). Instead of Magmar, Red has a higher chance of Ponyta, and instead of Electabuzz, Green has wild Raichu.
Japanese Blue is a mix of Red and Green. Sometimes it favors Red's common counterparts, sometimes it favors Green's. It does have a few distinctions though. You can find wild Ditto in Mt. Moon, and Tauros are no longer found in the Safari Zone (and I think Kangaskhan). You get one through an in-game trade. Meanwhile, Jynx, which was exclusive to an in-game trade in Red and Green, can now be found in the wild in the Seafoam Islands. Lickitung, also only accessible through an in-game trade in Red and Green, is now found in the Safari Zone. I think there's an in-game trade for either a Machoke or a Graveler (or both, I can't remember) that gives you access to their evolutions. Golem's the only Pokémon minus Mew that can't be seen in Red and Green without obtaining it yourself.
