LogFAQs > #930029019

I'll use Gen 1 for an example, since I didn't really watch the tv show after gen 1 and played every generation retroactively starting with Gen 5. From growing up when Gen 1 came out and hearing everybody around me talk about it constantly, I'll say.

They kept iconic first gen Pokemon like Butterfree, Pikachu, Charmander, Clefairy, Vulpix, Meowth, Onix, Gengar, Koffing, Gyrados, Lapras, the Eeveelutions, Snorlax, and Mew.
They kept Machop, one of the most popular Pokemon in Japan. They kept Farfetch'd because they gave him an evolution. They kept some unique Pokemon that had strong, fringe fanbases like Hitmonchan/Hitmonlee/Ditto/Mr.Mime.

And they took out stuff like Jynx, Beedrill, Fearow, Dewgong, Drowzee, and Voltorb that most people don't care about.