Quoted By:
What are some of the most underused Pokemon by Game Freak in main games?
Glameow/Purugly surely is one of them. The only game where you can catch Glameow in the main part of the game is Pearl, not even Platinum, and its only obtainable after 5th badge.
You cant find it in any other game, at least until post game
Another one is Pineco, avaible only in Johto and only via hedbutting trees and even then its rather rare
Other examples? (Dont really count 5 or 6 gen mons because they're fairly new)