It's pretty simple -
The name "Thor" cannot be copyrighted, BUT the visual look, and background story that Marvel created for the character can still be copyrighted.
Basically, Marvel's Thor is based Marvel's own "superhero" interpretation of the norse god...Since Marvel was the one that...
It all depends on the story you are making sometimes the normal "humans, elfs and dwarves" just don't work. I myself don't care much for writing about humans, since I see them daily, but elves, drow, and dwarves can be fun. Creating your own world allows you to control all aspects of the readers...
I have created my book starting with the universe, to the solar system, to the planets, to the planet, to the continents, all the way down to the tiniest of animals/monsters. Each region starts with 50 different animals/monsters, and as the book progress it increases, by the end I have around...
MC1 - Piscao Te'Lop (race-Siahmin) in his Normal Form. Standing at 6'10", 400lbs, with black beaded scales, trunks for arms and legs, a long tail and golden eyes.
War Form same as normal except adding on Brilliantly white Plates, spikes, and armor. Imagine a everyday lizard gaining the armor...