It's really good that you're asking for critique on the lighting because really, this is where you can improve the most by far and it's a good start that you're aware of this.
The first thing you need to realize and remember is to not shade each and every single shape separately. Your beast is one single, solid mass in space and the light will affect it as such. Muscles will become apparent in light, certainly, but only there where they truly transition into shading, not everywhere randomly. What I think you need to do is train your eyes and brain to truly see your creature as a 3D object in space rather than a bunch of lines you need to color. A good exercise for you would be to paint simple geometric forms without the use of lines. This will help you understand how a 3D object works in light and in space. Here are some references for you to work with: [link] , [link] and [link] . One very important thing for you to remember is that shading is at it's darkest where it starts, not where it ends. This is because, in a bright light source, everything becomes a small light source of it's own as the light bounces off in every direction and shows up again in shaded areas; this is why, in the shaded areas of the objects, you can very clearly see the red of the floor reflecting back onto the object. Do know that this reflected light only shows up in shaded areas, never in lit ones.
Either way, to try and help you out with this particular painting, here's a quick paintover. I hope it helps to make clear what I've been trying to tell you. Oh yeah, and one last thing; don't be afraid to lose detail in your shading. Trust me on this, ninety-nine out of a hundred times people will prefer a more simplistic image with proper lighting and shading over a picture with lots of fancy detail and shinies but with wrong shading.
continuing on lighting, (the paintover was great! i wish there was a tool to do that natively) you've used a heavilly diffused light for a composition i'd suggest a harder light for. the creature seems to be looking up toward a light from the bottom of a cave, using a hard light and harder lines between where the light is hitting the creature and the shadows will give a better idea to the viewer of exactly what the shape of the hole above is.
additionally, don't forget the environment, lhune's paintover did a great job of showing the creatures form, but left out where the light was hitting the ground around the creature. even in a diffused light, with an overhead lightsource like this, the shadow on the ground should still be pretty strong.
BRAAAAAAWK! this sound efffect serves no actual purpose save to add some troll-flavoring, im all out of allspice.
i hope some of this was helpful to you, remember to spend the time critiquing others in this group, it ultimately helps you more than them!