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