As a creature designer, myself, I can't say I've ever encountered a perfect creature that wasn't already from the real world. See, when we design creatures, for media of some such thing, we are creating a superficial entity. One which exists only in an illustration. We can create as much story and history to that creature as we like whether that's through design or through the action within an illustration. Nevertheless, that creature does not possess any kind of evolutionary background, and often exists just to be present in a game or book cover for the sole purpose of being killed or interacted with in a game. This means it can have flaws. Most creatures present in the world are evolved in such a way that they are perfectly adapted to their environments (unless you're a panda, of course, in which case you have adapted to be the most useless, stupid and pampered animal in the world). When I design creature's I'm all too often basing them off of existing animals. Taking elements I love and applying their properties to my creatures. My inspirations often come from raptors such as osprey, peregrine falcons and kestrels, or from beastly reptiles in the vein of the saltwater and mugger crocodiles. I love looking at insect life for it's incredibly variety and the underwater world has provided a myriad of different shapes and colours. Sharks and octopi are both incredible animals and perfect hunting machines.
If you want some of my favorite examples of creature design I can give you some samples of excellent use of the principals of creature design applied with great rendering and an excellent sense of storytelling:
LD Austin [http://www.creaturespot.com/main/2010/12/19/dragon-and-deinotherium.html]
David Melvin [http://cghub.com/images/view/88655/]
Andrew "Njoo" Hou [http://njoo.deviantart.com/gallery/?offset=0#/d2r3yjw]
Kekai Kotaki [http://www.kekaiart.com/]
Simon Dominic [http://www.painterly.co.uk/v_gallery.php?g=1]
Allison Theus [http://www.creaturespot.com/main/2010/10/27/first-post.html]
You can always check out awesome professionals sharing their personal creature work at:
www.creaturespot.com (Run by Davi Blight)
And you can check out the weekly creature design competition at ConceptArt.org "creature of the week" here:
http://conceptart.org/forums/forumdisplay.php?f=81 (Run by Clark Miller and Simon Sweetman (me))
See ya' round, and you can always PM me about creature design if you have questions or something. I'm not a professional, but I know a decent amount.