It's funny but I remember the original post that coopted Pepe. It was during the Trump v Hillary campaign when she referred to his supporters as a basket of deplorables. The meme emerged based on the Expendables poster with Trump flanked by the team he then had. And because it was 4Chan, Pepe was thrown in there.

I always found it kind of random. There was a thing on Chan at the time where Chloe Metz's face would be shopped onto female body builders. It could have just as easily been Metz, or Wojack, or any other thing that was happening Chan at the time. But it was that. And that's how a funny frog became a hate symbol for a while.