It takes a number of times enjoying Princess Mononoke to realise that there actually isn't a princess in the film and a clear dearth of anyone by the name of Mononoke. The title refers more to a concept and with a western upbringing "Mother Earth" would probably be the closest interpretation. And knowing that, the film makes an awful lot more sense however now that you know, you should watch the movie as I'm not going to explain it here other than to say it's an incredible creation.
The inspiration for the setting of Princess Mononoke are the woodlands of Yakushima, an island lying to the south of Kagoshima off the coast of Kyushu (one of Japan's four main islands). The ancient forests and thirty plus mountain peaks over 1,000m provided the Director Hayao Miyazaki with the identity of the feature though in reality Yakushima has been all but replanted following extensive logging over the centuries which finally drew to a close in the early 1960s.
However, if you do have the opportunity to visit you'll not only see deer with monkeys riding their backs (full disclosure: I've never actually seen this but am told it happens), an old tree, my friend's bistro and quite a few imported tanuki, but also the occasional sighting of an H2 rocket launch from the shores of nearby Tanegashima. And for that you need to watch a different Miyazaki movie, Howl's Moving Castle. Something that is also worth taking your time.