Pandora has a luxuriant and extraordianary wildlife, in every way. Its atmosphere having a high rate of hydrogen cyanide (HCN2), pandorian life had to adapt in order to survive. It has probably evolved in a way to support this mortal gas.
Animals on Pandora, like animals on the Earth, acquired their properties thanks to two factors.
The genetic drift is the first one. It corresponds to random variations of ADN from a generation to another, in a population.
The natural selection, as for it, corresponds to the easier survival of the individuals which are the best adapted to their environment. The best representants of a species survive, can breed and offer their
advantageous properties to their descendants.
Here, we will dwell on three of these animals, explaining why it is possible that they obtained some characters. So, we will see, by relying one the previous paragraph, if these creatures are realistic.
Na'vis
This humanoid species is the most advanced intellectually and culturally on Pandora. Na'vis have conquired the entire moon, have adapted to most of the Pandorian ecosystems and are an integral part of these ecosystems. Therein, they are very close to the neolitic men.
A Na'vi is about 10 feet high and weigh about 350 pounds on average, for an adult. Nevertheless, they are agile (in particular in the forest, what is shown to us in the film). It's understandable ; gravity on Pandora is lighter than gravity on the Earth.
Like humans, they have two eyes and four members, as well as a nose and ears feline in apparence. Their skin is blue. Their long tail is prehensile (it allows them to catch objects).
Their bones are strengthened with carbon fiber, as most of Pandora's animals. Na'vis are biped, so each leg supports half the mass M of the entire body. The undergone tension in compression by the femur has to be inferior to to a breakage tension Tbreak, whence : ½ M/S < Tbreak where S is the section of femur and Tbreak is common to all pandorian mammals, because it depends on the caracteristics of the bones. The Na'vis, if the constitution of their bones was similar to ours, should so have huge bones to support 160Kg while being filiform. Their bones confer them an increased resistance that allows them to be thiner and lighter. The fact that their bones are constitued of carbon fiber is questionable and rather unrealistic in a terrian point of view.
So they are bipeds. This is quite realistic because they they are really similar tu humans, in a morphologic point of view. We can think that we and them had a similar evolution on how to stand.
They have differenciated reproductive organs (males/females) and reproduce in a viviparious mode (similarly to humans).
This species has probably evolved culturally and intellectually similarly to humans. However, their civilization is at a prehistoric stage. They're able to make efficient tools. They are hunters-gatherers who respect nature and protect it. Na'vis have their own language, plus their culture, religion and hierarchy.
Indeed the Na'vis venerate a goddess ; Eywa. Eywa seems to be a collective conscience that unites the majority Pandora's fauna and flora.
This intelligence has been allowed by standing ; like for humans, it has allowed an enlargment of the cranium, and a larger brain.
They are also provided with a special physic particularity ; a neural queue that look like braided hair that allows them to be connected amongst themselves and with the rest of the pandorian fauna. It is in fact an extension of the nervous system. This organ seems to be particularly dedicated for pleasure during reproduction, such as a feminine clitoris.
Banshees
Banshees actually don't need to chew because there is no risk for food to go into the breathing canals. Their teeth (that are although all identics) are just used to catch and maintain the prey.
They live and hunt in group. Their pectoral muscles are overdeveloped, that permit them a really large and powerful flapping when they fly. Like other animals of Pandora, banshees use the low gravity and air density to move very easily.
While forest banshees stay in their territory, mountains banshees have developed a peculiar communication with Na'vis, in addition to their permanent connection with the collective consciousness that reunite all the pandorian life (Eywa). In fact, to reach adulthood, Na'vis have to tame a mountain banshee. This one will quit his natural environment to live in the Hometree, next to its rider.
They can be counfounded with another species, the Leonopteryx. This animal eats banshees. So, it is at the top of the food chain. It participates to the natural selection by regulating the banshees population. It is very similar to them but it is bigger and very rare. We can guess that it's rarity comes from the fact that it is red, and so it can be easily seen in the sky by its preys. Most of the Leonopteryx have so difficulties to eat, and just a few of them survive.
Viperwolves
The impressive jaw of the Banshee
We can distinguish to races of banshees ; mountain banshees and forest banshees.
This animal, which is so peculiar and representative of Pandora has four wings colored with complex patterns and made of a leather membrane outstretched on a bone structure, which is strenghtened in carbon fiber. This feature bring the banshees near terrian bats.
However, their wingspan varies from 11 meters (12.0 yards) to 15 meters (16.4 yards) for mountain banshees and from 6 meters (6.6 yards) to 8 meters (8.7 yards) for forest banshees. These ones benefit of their small size when they are hunting. In fact their efficacity has not to be compromised by the forest obstacles, and they have to be able to track their prey in small spaces. In a forest setting the smallest banshees ate better, so they survived easier. Although, forest banshees stay close to the floor, near the trees where they live in countrary to mountain banshees which live in uplands of Pandora (for example the Hallelujah mountains).
Their jaw is able to disengage and have short sharp fangs made of obsidian (which is rather unrealistic because obsidian is made of lava which was abruptly solidified when it came out of a volcano).
The ancestors of the banshees were probably aquatic creatures, by looking on this jaw that look like the jaw of some fishes known on Earth. We can also note that the orifices which are used for the breathing process are located at the level of their neck. So they have independent canals for breathing and eating, contrary to humans. This explains their small teeth. In fact the chewing action is useful tu us because when food isn't in small parts, there is a risk for it to invade the respiratory tracts.
Viperwolves are hexapod brown/black animals. Their mat skin is a camouflage for them, in the pandorian forest. They have a thin chest but they have powerful muscles. They can so track preys on long distances. These animals have differenciated sexual organs and are viviparous, like the majority of the pandorian animals. There green eyes, that have probably the same rate of rods (photoreceptor cells used in a dark environment) and cones (photoreceptor cells used in a bright environment), allow them to see very well during days and nights (admitting they have similar photoreceptor cells as us). Viperwolves can also recognize simple forms, they perceive depth and are capable of complex skills in communication, apart from their neuronal connection to the collective intelligence of the moon. Their six opposable thumb give them a great agility and allow them to climb in the trees easily. Their olfactory senses are extremely developed and allow them to smell a prey at more than 8 km ( 5 miles ), contrary to terrian wolves which are capable of smelling an animal at 2 km ( 1.2 miles ).
Precisions on the sens of smell ; odors are in fact molecules caught by a set of cells located in the nasal septum. They send nervous signals to the brain that decodes these informations to reconstruct the corresponding sensations. Humans have about 5 millions olfactory cells besides dogs and wolves have a 35 times more developped sense of smell (with about 175 millions olfactive cells). Viperwolves have the capacity to smell a prey 4 times further than dogs, that to say about 600 to 800 millions olfactive cells. Their muzzle is too small to welcome all these olfactory receptors. So their nasal septum could continue further (for example in paws or ears) or have more effective olfactory receptors. In any case, a such smell is surprising and unknown with terrian mammals.
Viperwolves possess jaws that allow them to apply a pressure of 4 kg/cm2 (9 lbs/inch2). We know that humans can apply a pressure of 57 kg/cm2 (126 lbs/inch2) and wolves can apply a pressure of more than 150kg/cm2 (331 pds/inch2), so this affirmation isn't really realistic...a so few powerful jaw just allows them to eat small animals that have a very thin body like insects or little mammals. But most of the pandorian mammals have a skelleton reinforced in carbon fiber, a parameter that could sharply reduce their diet. Another hypothesis is that with the pandorian gravity, that is very low, their jaws do not need to be powerful. But the gravity should be really really low. Actually it is a 0.8g gravity. Earth's gravity is 1g, so the difference is small and negligible. Their jaws aren't very realistic, unless viperwolves only eat small insects, that is impossible because they are carnivorous.