No one really knows exactly how the METE started.
A few theories went around in the wake of it among what was left of humanity and the new equine society that now lived across the planet Earth. Some people believed it was aliens, others believed that it was some powerful magic that was unleashed. But whatever the reason, it completely changed life on earth as people knew it, and so far there is no way to return it to normal. But what happened you might ask.
It began with strange news reports across the world of people spontaneously transforming into members of the Equus species, or horses and their relatives as they were more commonly known. However, as this happened many of the people speaking of it started to transform as well, and those watching it and in surrounding areas became equine as well. Their minds and memories re-written to make them think they had always been like this. Some of them retained human level intelligence, others even retained a basic human shape, but most of them went completely feral.
Today, society has completely devolved into a more agrarian society with ruins of old human cities in the process of falling apart. The Anthros and the intelligent horses have banded together, with stories of human society becoming little more than myths. However, some of them retain memory of the METE as its become known as, and they seek answers as to what happened, even if they don't want to be human.
Humans on the other hand are still around, mostly surviving in underground bunkers or in isolated areas. They have developed ways to protect themselves against the effects of the METE as entering the forbidden zones as they call them will transform them into equines without them. However their protections have no effect on getting close to a horse, which will transform them automatically.
Equine society is divided up among Ferals, Savants, and Anthros. Ferals are like the horses of the old world, completely animal in mind and body with nothing resembling human intelligence. Savants retain their feral bodies, but they have the intelligence level of humans from before the METE. Finally the anthros resemble equines that walk on two legs and are as smart as the Savants. They can also be any species of equine from before the METE, and they are capable of having viable offspring unlike pre-METE.
What happened to Earth on the day of the METE? No one knows, and there will always be people, and equines, seeking answers.