It all started with a strange stellar object entering the Solar System. Officially scientists called it a comet, but no one's entirely sure. The object entered the earth's atmosphere and exploded, which sent out strange waves of energy across the world. Scientists dismissed it as nothing more than an odd effect of the object and everyone just let it pass.
At least for a few days until the full effects of the wave started to be felt. People started to go missing, but there was a large number of horses and other equines seeming to appear out of nowhere. By the time it was discovered that the new equines were all transformed humans, it was far too late and the last of humanity became equines.
There was nothing left of humanity at this point, and while there was a lot of blame being thrown around, there wasn't much that could be done because of the lack of opposable thumbs. The new horses maintain their human memories and personalities as well as can understand one another, and while there are scientists studying it as best they can, there is no solution found.
So now as there seems to be no solution, many have reached the conclusion that there is no cure and are getting settled into the life of their new species. Likewise, animals that were born equine have had their intelligence increased by the same effect. Curiously, it seems they maintain their human lifespans as well. Some have speculated that the effect may even make hybrids genetically viable, but its hard to tell.
Species humans have turned into: Horses, Ponies, Zebras, Donkeys, etc.