Pepe’s been a meme for damn near ten years. He just somehow became the most popular one for remixing, and then that became a meme with people trying to make “rare Pepes.”
My normal friends started mentioning him two or three years ago when he started showing up in their Facebook feeds.
Pepe is not really a white supremacist meme, he’s just the most popular meme, so he gets remixed by them, too.