It is an adorable picture and very well drawn. The expression on Opine's face conveys his emotion brilliantly. The subtle shading is well done, enough to add depth and shadow to the image but without being excessive. Possibly could be improved by adding shading to the further away legs, but otherwise is great.
The ponies themselves are well drawn, especially Opine.
In the background, there is a slight imbalance, with a large empty space on the right, but it is not too detrimental. The tiny pony ornaments on the tree add a very nice tough, incredibly cute. The fire is a little bit straigh-up-and-down, which is odd, but a very difficult thing to draw and otherwise very good. I particularly like the little bits of glowing ash sparking out the top of the flames. The christmas tree looks fantastic, the pine needle effect is great. However there is an odd branch with a large gap underneath it which looks a little odd.
Overall it is a fantastic piece of art with a lot of care and effort put in. I commend your skills.