So lately I've been wanting to get back into designing creatures, and by creatures I mean anything really. So to this end I'm asking you guys and girls to give me some ideas.

The idea is simple. Give me some guidelines for a creature! Want to see a parasite evolved to live on a low gravity gas giant? Or how about a cave dwelling creature that locates prey purely by smell? Give me some ideas and guidelines. Be as exact or vague as you want and I'll try to design something that can exist in the environment specified.