I am not sure, how lower gravity would allow people live higher up a mountain (it is not gravity, it is a lack of oxygen which limits the habitability in that dimension, and the lower the gravity is, the less atmosphere the planet is able to detain, but I am not a physicist, and could be mistaken), but in a different gravity world both flora and fauna would adapt to it--this might be a question for the worldbuilding exchange--the trees would likely grow higher, and yes, larger animals would be able to fly, like dragons and pegasi (is that a correct plural for Pegasys?).
It is for you to decide if the strength of humans and ordinary animals would be scaled down proportionally, and that would dictate the way how to show it throughout the story.
But it definitely has to be showing, not telling--mention the trees, touching the clouds, a bird-of-prey with a six-foot wingspan, etc.
At the end, it is your story and not the gravity which makes the reader interested.