I've noticed in some of your art pieces that there are some sections that have shaky lines. I'd recommend focusing on fixing, and trying to notice if you have any, before anything else.
Your shading is perfect, your shadows could use a little work (I won't be too harsh on them, shadows are a bitch to do). Your poses seem very natural, but you should try to do full body shots a bit more, rather than just heads and hooves.
Your expressions are adorable, and full of life, so you need no other help on that. Backgrounds tend to be very creative, but I also recommend doing something solemn (just once to show dynamic between character and atmosphere).

I have way too much time on my hands to notice these things.