A term given to Pokémon "fans" who only enjoy the original games in the series. Many of them were kids in the late 90's and grew up playing Pokémon Red, Blue, and Yellow Versions on the classic Gameboy (as I did). However, their negative reputation comes from their constant, idiotic criticism of the brilliant innovations made by more recent games in the franchise, particularly the new Pokémon themselves. Some Genwunners have even gone so far as to deny the existence of these Pokémon, maintaining that the original 151 are "the only ones that count". They also often complain that the designs of the new Pokémon "aren't as creative." A single look at some of the newer Pokémon makes it easy to recognize that a solar moth deity, flying gargoyle-shaped scorpion, and an antimatter-controlling basilisk might just be a bit more interesting to some people than a pile of sludge and some eggs. Basically, Genwunners are just biased, nostalgic idiots who can't understand that things need to change in order to progress, and that children growing up these days might see things differently.
Sensible Pokémon Fan: "Hey, did you see the new Mewtwo forme? I can't wait for Pokémon X and Y!"
Genwunner: "WHAT?! How DARE they desecrate my childhood! They've turned my precious Mewtwo into an alien! This is EXACTLY why these games have been going downhill. They should have just stopped after the original three. Anything that comes after is SHIT."
Sensible Pokémon Fan: "Are you kidding me? You're saying that something is bad just because YOU didn't grow up with it? Grow a fucking brain. Every Generation of Pokémon is great!"