All in all, this is a very beautiful piece of work - which is astounding to look at. The design has been well drawn and coloured and the pose of the mare is stunning.
I must say one thing though, in order to give Nala a more 3D like effect, maybe add some dark shading to help the image stand out more instead of just the light on its own. If you look at the image of a horse you should see where the areas of darkness lie - so it should help.
But basically this is gorgeous, the effort that has gone into this is awesome, if you keep the criticism in mind I think you can improve your abilities further.