Like, Rainbow's not a fillyfooler at all, she looks like a secret descendant of Rainbow Dash, both have Rainbow manes so is the tail, but the Changeling is probably the Pony's past that she was a crazy, serial killer of the Changelings, like from all she sucks blood, she's carnivorous, and due to that, she was wandering out of the Badlands with no company then she saw some guards therefore turned into this pony, but when she went up to the guards, the guards caught her almost biting them, they only stopped her with a sleeping spell. Then she was sent to Celestia to change her mind to stop on being carnivorous then she also became her true self as her great grandmother, a pony. It's only a theory and I know I may have mistakes with this, don't lecture me with some facts from the series or fandom since this is only my own opinion.