Tengen Toppa Girdle Lagann

Walls, maybe. Looks like it would make a nice fence, though! And it isn't very poorly-drawn, you seem to have a good handle on foreshortening and paneling. That just leaves the quality of the lines, but the rest of the piece has the same quick-sketch style, so you're pretty much covered there too : )

If you were going for show-accurate, the characters need some work. However, if you're making your own style, they look pretty neat! They are both immediately recognizable, and the expressions are accurate and fit the situation. The only things to improve are the shapes of the hindlegs and the muzzles. Rainbow Dash's in particular have a strange look. The hindlegs...actually, after looking over your more recent drawings, you've gotten those figured out! Good job ^_^ For the muzzles, they can probably both be shortened about a third of their total length and still keep with the style you're making; they are tricky in the FiM style because of how, on mares, they don't look like normal horse muzzles (males are closer to that horse-y look). If you try to make them long on mares, they start looking like canine muzzles, so keep them short-ish.

The only other thing with this picture is RD's foreleg, and pony foreleg posing is always tricky. Reference pictures are very useful for figuring those out.

I've looked at a few of your other drawings and you are showing improvement, so keep up the good work!