A man wakes up to find he's inhabiting the body of a cartoon horse who wasn't exactly Mister Popular, judging from how shocked the locals are when he says things like 'please' and 'thank you'.
Long story short, humanity thought it was a good idea to kill itself not with pollution or nukes, but with bio-weapons. I was the lucky one as I was somehow resistant to the changes everyone went through and only benefited from them. She wasn't sadly
Nightmare Moon banished Celestia, only to be defeated by Ahuizotl. Ponykind were made the scapegoats to blame for her scourge. 1000 years they pray for a saviour to deliver them. Anypony will do. Enter the last hope, a pizza delivery dude.
In an Equestria where Celestia and Luna have slain each other, a human is brought to Equestria in the body of King Sombra, in hopes that he might be able to save the kingdom from the fiends of Tartarus...
A human soul is stuffed into King Sombra's body. Having no idea what's going on, and thinking he is in some kind of coma, he just casually strolls into the Crystal Empire.
Far from Canterlot and forced to work in gem mines for cruel Diamond Dogs, a young unicorn named Bone Marrow's pursuit of his cutiemark reveals a unique talent: raising the dead.