I get the butterfly, sort of. Why would Scootaloo get a cutie mark of an animal that starts out life entirely ground-bound, but goes through an amazing metamorphosis where it gains the ability to fly? Yup, can't think of any reason there.
Would also indicate Fluttershy being involved, which could be interesting. Maybe Rainbow Dash sees a parallel between Scootaloo being a weak flier (at best) and what Fluttershy had to go through due to her lack of flying skill as a filly. Could be a fun story, and I could certainly see Fluttershy being able to teach Scootaloo a few things Scootaloo lacks. Like patience.
Of course, hilarity ensues when Scootaloo tries to make her own cocoon as she didn't quite get that Fluttershy was talking in metaphors.