I'm realizing that, when I wrote about design last, I might have seemed like I was downplaying the role of understanding anatomy, zoology, ecology, etc. - that was not the sum of my intention. Keep in mind as I talk that these are just my personal observations - creature design is not just about making awesome creatures: there are several forces at play, not the least of which important is public reception. In my perception, there are at least 5 things going  on within a good creature design:

1) Good understanding of anatomy, biology, ecology, climatology, etc. --------Science!
2) Good command of visual art principles in whatever media you're rendering in ---- Art!
3) Hybridization and generation techniques ----------------------------------- Design!
4) Understanding of how the creature will function within the final product -- Aesthetic!
5) An ability to willfully and purposefully function outside of convention ----- Originality!

I'm not saying that these things are absent within the canonized creature designs we all know and love, nor within our own collective work! In fact, each of us has a very different take on creature design and there are definite schools of technique, thought, and genre within our various circles. What I am saying is that each of us brings something individual to the table, and I feel like there's a common technique we all use to design creatures. This is one of the best descriptions I've seen of it from IRIRIV, and I think it's a great example of each of these 5 elements coming into play: [link]

As we set about designing creatures though, let's talk for a minute about backwards design: the idea that we use the goal to create the product. Within games, movies, etc, the most important aspect of the creature is often not inherent in the concept, as strange of an idea as that may sound! The most important aspect of the creature is its functionality - it's integration into the aesthetic and the theme of the product. I see nearly all creature design done from the bottom-up (imho), and I think that's in part due to a scientific approach to creature design. We assemble creatures from other parts, we build creatures with a functional biology, and there's nothing wrong with this approach. But, how much of it is necessary? In part, its due to a desire to make things look real - from that point of view, it's 100% necessary - good CGI is going to show rippling muscles as the creature runs, realistic weight distribution,  realistic adherence to gravity, etc, etc! It's going to need the rib cage appropriate for the heart and lungs that allow it to function for its physical frame!

My concern doesn't lie with that, at all. Where it does lie is with, specifically, aliens. Let me bring up Super-8: one of the best movies I've seen in a long time, but with really really poor alien design. I'm not pulling punches here - I hated that thing, but only because it was a gorilla-spider-pus. Where's the concept of alienness? I have the same problem with Avatar, as much as I loved that movie (as much as everyone else, according to money!). Those creatures were designed from the ground up - biology up to creature - and they totally fail to read as alien. However, et me bring up what I think are some good examples: Alien, and Expedition. Even though these both have some element of biological design, in both cases there is a healthy level of skepticism or ignorance about how the actual biology functions.

As we design creatures, it's important to be aware of the audience we're designing for - the general public. We have to hit this 'canny peak' of appeal, getting the perfect balance of the strange and the familiar that will allow people to latch on mentally and not automatically reject a concept that is 'too alien.' However, as we all know it's a terrible idea to prematurely reveal the alien or monster in a movie - it ruins that element of suspense. Why not design a creature that is hard to perceive at first, and allow the natural perceptions of the audience to shape a natural reaction to a truly alien creature? There was some element of this in Attack the Block - overall the creature design is pretty standard in the beginning, but it moves on into the realm of bringing up human perception. When you first see the alien's glowing eyes - pay close attention to what happens next!

When I design creatures, I find it extremely hard to get out of this 'design-from-biology' mode - it's simply an underpinning of human knowledge that we understand the world based on our current level of perception! For a good example of this, think about how hard it is to create a creature not designed on the terrestrial HOX bodyplan! Tom has done a bunch of work strictly about this concept: remember that robot that flies by changing shape? Tom predicted it fully back in 2008! [link] These things are simply programmed into us by our environment, so that we understand body and motion in distinct ways. another example: [link]

One trick to breaking yourself out of this pattern is to get out of that realm of functionality - get above the realm of biology. I don't mean totally abandon it, because you nearly never can (I know, I know, energy beings). But as I'm sure many of you know, science still isn't sure what the function of various tree hopper bodyforms are for: [link] - I think it was Rachel that mentioned that, in fact, that's how we go about trying to explain the world with science: observe and then explain. As we design creatures, I would say there can be a missing step of 'aesthetic first' - the whole thing coming before the biological explanation. What I'm saying is, especially with aliens, let the design be supported or tweaked by biology after the fact. This, in many ways, is similar to illustration principles of abstraction -refining or exaggerating details of a design for emphasis - there's a key skill there of knowing how far to abstract something from its original appearance for maximum appeal.

tl;dr - I go through a process of design evolution to maximize concept and appeal, and at the most basic level the process is this: start with a simple design, go for maximum impact with the shape and aesthetic, and redraw it 20 times - each iteration being informed by the previous iteration. You don't need to do 20 steps necessarily, unless you want to go from a very simple shape to a full creature, but try it out and see where it takes you.