This was originally gonna be a Fell Beast/Nazgul from Lord of the Rings, but I decided to change it's head/face to that of a Dunkleosteus and add the flames to create a Dragon/Phoenix hybrid type creature .
I love the colors here, the purples and oranges work really well together. The fire on the creature's back stands out nicely against the blue in the sky above it. really creative design, too.. it's always fun when you start drawing one thing and end up with something completely different.
The one wing getting clipped off on the side makes the dragon look a little bit crowded, I think. His face is so close to the edge that it gives the piece a little bit of an uncomfortable feel. I'd suggest adding just a little bit to the canvas on the left side, enough to get the tip of that wing on there and give the little (...big) guy some breathing room. The horizontal line of the mountains is working really nicely to draw the viewer's attention to that area, so it's kind of startling to hit a "wall" so close to the focal point.
The scale is also a little bit confusing at first glance. At first, I thought it was flying high above the mountains and very close to the viewer, but now that I sit here looking at it more closely, I think he's extremely large and flying right above the mountains, right? I think maybe defining the shadow on the ground a bit would help - at the moment it looks like shadows in the mountain crags. Maybe making it match his shape a little more obviously would help to draw it out a little.
All that said, this is a really nice piece, I like it a lot. That is definitely one dragon nobody will be riding anytime soon.