This map features a world with a very minor difference long ago, that built up to what you see here.
In our history, the Vikings visited and possibly set up shop briefly in the New World before being forced to leave. Here, they stay a little bit longer, and manage to have a few horses and farm animals escape. By the time other Europeans stumble upon North America, the horse, pig and even dog are prevalent. As are nations of developed peoples.
The Iroquois, or Haudenosaunee, are one of the strongest represented here.
This map took possibly the longest of all I've ever created; somewhere between six months to a year.