15 years ago a meteorite struck Equestria.

In truth, it was an ancient wreckage, blasted off from some battle fought so long ago that the successors to the successors of those who could even remember the race that fought it, had perished, and vanished into dust.

Inside the ship something still lived, even as all of it's brethren dried out into fossils, it stumbled out of the wreck and into Equestria.