I wrote up a post for the Animation Mentor blog regarding my thoughts on creature animation. In short, the reasons why I think it's important and what you should look out for:
1. Most companies feature creatures in their movies.
2. Diversity in your reel.
3. Root the performance in reality.
4. Get the weight right.
5. Interaction between creatures.
Head over there to read up on all the details. I hope it's helpful!