One of my recent favorite movies is James Cameron's Avatar, and while a lot of physics is a bit sketchy, some of it makes sense.
The moon/planet the movie takes place on, Pandora, is a lot smaller than our earth. This means that gravity isn't as strong. So, while humans have to learn to deal with the debilitating effects of low gravity, the natural life has had plenty of time to evolve.
All of the animals and plants are much bigger than their Earth counterparts. The people that live there are much taller, as well as the trees and plants, since they've had less gravity to fight against as they grew. Most animals have six legs, to give them more traction on the ground and keep them moving at a decent speed.
Pandora's low gravity also allows for the existence of banshees, dragon-like creatures strong enough to carry people around to hunt or in battle.