I've discovered a major problem plaguing beginner artists is that they're often under the impression that knowledge of anatomy or nature is not required for the development of the fantastical and alien. Anyone who has ever compared drawings they've made as a 10 year old with their current work can tell you this is false. You see improvement because you've learned how to observe nature, and you've learned anatomy. A bird looks the way it does because that design functions. We don't see giant birds with tiny wings or wings sprouting from their neck because it's inefficient.
It's important to not let your imagination become restricted by strictly following the rules down to the math, but it's equally important to learn from nature, so that you're crazy critters can be wild and fanciful while also being believable. The same rule applies to the design of machines, robots, weapons, etc. Any design can be made better with a tiny injection of knowledge and research. And if that's not enough incentive, do it for your own integrity. Who's going to believe your design if you yourself don't even understand it?
I find this is often an issue when people draw dragons. I hear it all the time, "It's a mythical creature, so I don't need anatomy". If I go to watch a fantasy movie, and it features a giant fat dragon with tiny slashed up wings carrying it around in the sky (for example, the dragon from Shrek), then that creature must be intended to be comical. Because it certainly looks funny flying. Realism is often tossed for the sake of comedy. The outrageous and improbable is funny. But if you want to show me a serious movie with dangerous dragons, then they better not share those traits. If they do, then the design fails because I'll lose my sense of immersion.
An example of a movie with dragons that does this successfully is Reign of Fire. (http://www.milesteves.com/gallery/d/1108-2/ROF-crouch-6.jpg) The dragons in this movie are very big, but they are convincing to the eye because the wings are even bigger, and the wing membrane attaches a long way down the body, giving the creatures a lot of support in the air.
Dragon designs that fail can often be found in videogames. For some reason, the dragon designs in games are often worse in terms of credibility than those found in movies. For example, look at the dragons found in the game The Elder Scrolls V: Skyrim. (http://www.skyrim5.com/images/skyrim-dragon-gliding-at-you.jpg) The main portion of wing membrane is found on the fingers of the wing, giving its body no support in the air whatsoever. To me, this looks immediately odd, and when I play the game, the dragon's flying animation looks strange because of its diminutive wings. Not everyone notices this though, and that's due in part to the fact that the general public does not understand how flying works. Wings are often thought of this magical device that can be slapped on anything to make it fly. But this doesn't give an artist the excuse to use them as such. Anatomy should compliment design, not detract from it.
Some semi related drawings:
Studies of a replica harpy eagle skeleton. Giant pectorals are important for achieving lift.
A page of dragon anatomy done for a Biology class assignment. Wyverns, (four limbed dragons with wings and hindlegs) are easy to create the anatomy for, because it already exists in birds, bats, and pterosaurs. Hexapodal dragons however present a fun challenge. You need to create space for two sets of forelimbs, one pair forms the wings and takes up the bulk of the muscle room. This image was largely inspired by Todd Lockwood's DND dragon anatomy painting. The drawings themselves look small in comparison to the text to fit on the page, the image is very large however and can be downloaded for a larger view here: http://katepfeilschiefter.deviantart.com/#/d4i7aq5
Now for some silly unrelated human legs. These were drawn from some old high school anatomy class handouts that I still had lying around. I think they were actually from an anatomy colouring book. Surprisingly in depth.