Heya!
Nice idea so far!!!
I think the answer you are searching for can be answered with backstory about the character. What and why is he/she/it? Thinking this way will give you reasons for specific anatomy.
Generally speaking things are made up the way they are based on how they survive - Eating, finding/creating cover (home), fighting, procreation and expelling. Then I guess if they are more evolved pleasure, self expression and finally self realization.
http://en.wikipedia.org/wiki/Maslow’s_hierarchy_of_needs
Here are a few questions you might consider to get you started:
Close your eyes and think about -
1- What and how does he eat? ( does he chase it down or shoot it, mind control it? )
2- Can you see him sleeping? Where? Does he need to sleep?
3- Who or what does he protect? How?
4- Who are his natural enemies? The ones he runs from.And what does he need to run from them or protect himself from them. Who are the enemies he’s chosen? ( if he is a destructive thinking being - like humans:/ )
5- What’s his job? Can you see him doing this job?
6- What environment does this character come from? Is it a place like Earth? Does it have the same gravity that we do? How does it move? Like a snake, like a spider, like a fish, a sea sea anemone?
Again… most things look the way they do based on survival in their habitat. At least from a human perspective.
If they are thinking creatures like humans and they’ve moved past the basics, then they might be augmenting themselves in ways to support them in a habitats and mind states that are not the base norm. ( space suit, exoskeleton - http://www.youtube.com/watch?v=0hkCcoenLW4, parasailing, etc)
All this thinking should lead you to - does it need a skeleton? Spiders don’t have bones but they have a rigid exoskeletons. They have connection points. Kinda like hinges and ball and socket joints. We see their “bones”. Does he NEED muscles?
Your creature may not have human/Earth like limitations. They man have vast development in other controls that we as humans can’t even comprehend. Could be a jelly fish if it can control everything telepathically:)
The choice is yours of course, but you must choose.
You should make a mood board filled with all of your thoughts. It should be filled with images, statements, music and words that relate to your character and it’s environment. You will also want to pull reference of critters or objects here on earth that might have similar characteristics to your character. There’s nothing like reference to help with believability.
The cool thing is this type of thinking should give you everything that you need to create a an amazing memorable character - holistically speaking.
All of this can come after you lay down a cool sketch or it can come before. Doesn’t matter as long as you think about it at some point so you can begin to answer questions about anatomy and such. If you were on a job, believe me, someone would have very definitive ideas on what and why this creature exists and they would give you their POV. BUTTTTTTTTTT!!! Remember the most powerful/successful artists are highly thoughtful!!!
They set the agenda.
Good luck with your sculpt!!! Hope you find this somewhat helpful.