Some say that when you die, your soul stays with your body. Some say that your soul goes onto a better place, a place that's filled with peace and happiness.
But the ponies and bison of the desert know better. When a soul is wronged, when a pony or bison dies in cold blood at the hands of something truly evil, it comes back. It comes back looking for vengeance.
Those spirits are known by one name:
The Chindi, the Demons of the Desert.