The idea for this darker spinoff of MLD was based on Celestia's comment from the original story: "When I realized where you ended up, I expected the worst. I figured you to be ruined, tainted and tarnished from this world's cruelty."
Through an accident of circumstance Rainbow Dash is transported to another world. However, her first encounter with a human isn't a loving brony, but a cruel sadistic human who sees her only as a chance for a big payday.
Since the subject matter is darker and writing about filly abuse isn't my thing Rainbow gets transported as is with her memories intact. No cute little filly here.