It has been on Earth ever since the gods, and goddesses of ancient Greece ruled the land, and mated a lot, but what most mortals don't realize is that the water hole that now remains in a North American forest has an enchantment on it. It was before the continents were formed that made this enchanted lake exist.
Demeter, the goddess of nature, and fertility knew that one day that humanity would no longer rely on horses for transportation, and saw the horrors after the invention of vehicles beginning to pollute the air in the atmosphere. So to keep humanity away from death by air pollution, Demeter enchanted the lake. Now whoever touches the water of the lake will gain the lower body, and 4 legs of a horse, along with a contagious spread by acts of bestiality sex when they decide to make.