"The humans? They were here. This history book says so."

Shortly after the return of the Crystal Empire, Twilight Sparkle borrows a book from the Crystal Empire's library, and it speaks of Equestria before Celestia and Luna lived among the other creatures of Equestria, when they lived just as the sun and the moon.
But the book also speaks of a species that was banished from Equestria,

the humans.

Twilight, intrigued by her new found source of knowledge of a species she has never heard of before, decides to look deeper into the study of humans.