The version of the game defines whether some Pokemon are catchable or not.
In the Red Version you can find:
and you can't find:
which are only obtainable in the Blue Version.
The version also makes it more likely to find some pokemon in the wild:
- In the Red Version Weedle, Kakuna, and Beedrill are more likely to appear in the wild than Caterpie, Metapod, and Butterfree.
- Nidoran ♂ is more likely to appear than Nidoran ♀.
- On Route 8 you'll find Mankey, as opposed to Meowth in the Blue Version, though they're not exclusives to each version.
- PIkachu can be found more frequently in Viridian Forest 1.
- Though Pinsir can't be found in the wild, it can be bought at the Game Corner.