Equestria has discovered industry, and with it, the need for labour. Using their inherent gifts, the unicorns have taken power and occupied key locations in Equestria. Decades have passed, and the world has come back to a balance of sorts, towns ransacked and rebuilt as factories, farmland forcibly claimed, cities housing only the elite and their servants. The power struggle has ended, and the unicorns won.

EDIT: Alternate Universe tag added, though I prefer to think of Cogs as an extension of canon, it isn't the Equestria everypony knows and loves. Cogs does have the same history as the show, certainly up to Magic Duel. Beyond that is open to speculation.