Absolutely no way this is true. Acquiring and using horse or rat meat in the US on this scale would be cost prohibitive and logistically impossible. And if it did happen it would be plastered all over the major news networks not hidden in some obscure corner of the internet.
If you want to feel better about Wendy's or Burger King or McDonald's, they have some of the highest standards in the industry for their ground beef requirements. What's in the meat is NOT what you have to worry about. Think 16 year old kids and disgruntled mid managers cutting corners with basic food safety protocols...
I think Burger Street uses some kind of salty rat or other varmint. Not really of course, but it taste like a whole different kind of meat. And it's delicious.