James Camerons core idea for the Avatars fictional creatures was for them to be "superslick and aerodynamic, and be like a race car with racing stripes". Neville Page worked on Avatar as the lead creature designer. He, Wayne Barlowe (author, artist, and initial lead creature designer), and Yuri Bartoli (concept designer and supervising virtual art director) adapted Camerons conceptions of the fauna into a design that served three purposes: to appear expressive, to function with animation technology, and to seem realistic. He and creature designer Wayne Barlowe sought to base the design of Pandora's creatures on race cars, but they struggled to adapt the concept. Page drew on his education in automotive design, recognizing the irony that race cars were based on real-life animals in having "bone lines". Existing automotive designs drew from seashells, turtle shells, and insects, so the designers returned the design to the fictional creatures. They found that the prime challenge in designing most creatures was to give them organic appearances, including skin texture. Some creatures were also designed to have special breathing holes located in the trachea, copying how cars have intakes. Challenges that the creatures posed for visual effects technicians were to form "walk and run" cycles for six-legged creatures and to impart credible flying for creatures that had four wings.  Many of the animals also have four eyes, with an apparent major and minor eye on either side of their head.
The fictional creatures are not connected telepathically according to Cameron and the designers. However, even though they discussed the idea of the creatures being part of Pandoras "Worldmind", they preferred to interpret the creatures as having heightened instincts. Page explained, "Animals are hooked up to this planet. Were the ones who are detached.... The way I dealt with it was, We have so much rich [material] here to reference, that we don't have to dream up a whole new process of animal awareness."
The fictional moon has less gravity than Earth, so the creatures larger sizes match their environment. Most Pandoran wildlife is hexapodal, or six-legged. Much of the fauna and flora is bioluminescent, which is seen in creatures on Earth such as fireflies, many deep sea animals, and some microscopic algae. The aforementioned breathing holes, located on multiple parts of a creatures body other than the mouth, are similar to spiracles in some of Earths animals. The flying reptile-like creatures in the film can be compared to extinct flying reptiles such as pterosaurs and to the modern gliding lizard Draco sumatranus'.