It isn't a crossover. It isn't exactly original, either. Some folks call them homages, some call them pastiches, some call them 'style crossovers'. They can be about

• a magnificent international thief named Twilight Sparkle and her band of disreputable rogues
• the secret memoirs of the most-decorated coward of Equestria's golden age
• the cautionary tale of a mare who walked too far into the shadows in search of monsters - and the Elder Ones she found there
• the accounts of a meticulous and precise stallion who has dedicated his life to the intensive practice of deductive logic in a world of miracles and illogic.

The most salient aspect of this class of stories is that they take the template of a certain distinct type of tale, or a specific authorial voice, and cast those elements into the mold of pony, to find which elements are particular to their setting, and which are truly universal. These are not stories of characters from other worlds of fiction interjecting themselves into Equestria - these are echoes, reflections - the stories of other worlds retold in equine voices, of ponies whose lives are mirrored reflections of their other selves.