While making the main picture for this form, I asked my daughter for advice at various points. One of those points was on the subject of the tail. When I asked her if she liked the rainbow colors on the tail fins OR the tail spines, she said she wanted both even though my view was one or the other, NOT both. While the final version was to stick with dark blue tail spines and rainbow on the tail fins, I did offer to post her variation, since she helped me out so much on ideas. So, here we are.
 
Originally, she didn’t want the shine on the hand claws either, but she changed her mind at the last minute, making the tail the main difference.
 
Here is a Rainbow Dash dragon form.
 
This pic is a variation of this one, which is the one published with a fanfic chapter.  
Deviantart link: Dragon Form Master Mev  
Derpibooru link: https://derpibooru.org/images/2593345
 
I have every intention of this picture being the last variation I post for anyone noticing the other two around the same time.