Zettachrome

Rainbow Dash, the powerful mare bearing the Element of Harmony, through an unfortunate and unknown turn of events, finds herself in an entirely different world. But it's not a different world, only...a different time. She finds herself over a thousand years in the past, thrust into a prophecy unknown to her, into a war with the demons of Tartarus, and the weight of a world lays quivering weakly in her hooves. http://www.fimfiction.net/story/130956/skyclad-honor-bound-a-millennia-past