I'd like to see a little more detail on the horse, but I like the composition. The trees frame the horse nicely and I agree, the billowing tail is a great touch.
This is lovely. I like how he is runnining towards the viewer. The trees form a good frame around him. The bottom crop is a little tight. I would have liked to see a little more foreground and less of the treetops. The hazy loss of contrast is ok. It gives the pictre an early morning type of look.