Version exclusives can generally be paired up as equivalents (Oranguru vs. Passimian, Drampa vs. Turtonator), but this hasn't always been the case, particularly with respect to standalone third versions. For instance, in Red and Blue, Caterpie and Weedle were both available in each game, with one being much more common than the other depending on version. Then when Yellow came along, you could get Caterpie there, but Weedle wasn't just a rarity, it was removed entirely from all the encounter tables.
Yellow also locked you out from catching the same species that Team Rocket uses (Ekans, Koffing, Meowth), with Ekans also being excluded from Blue and Meowth excluded from Red, but Koffing's status as a version exclusive didn't have any prior precedent or counterpart. And of course there's the incidental fact that you're locked out of Raichu as well (as every Pikachu that used to be in the game is removed, replaced solely by your starter, and every Raichu except for Lt. Surge's is also removed).
