A lot of new writers ask whether their ideas are protected by book copyright. The short answer is usually no, but the writer is usually asking more questions than just whether or not of US copyright protects her ideas. They may be asking about the world that he or she has created, certain elements their story that no one else has such as an original creature or character race. So, how do you make sure that all of the elements that you come up with are protected so that people don’t use your in the work in the future and therefore diminishes the integrity of the work that you’ve written yourself? That’s exactly why people worry about copyrights and rights that they hold to their worlds that they create and various other elements that make their story totally unique.
You don’t have to worry too much about the world that you create. If you create a compelling world with enough unique elements to where people are going to recognize it when they read it, then you don’t have to worry. For example, take the world of Harry Potter. You could send students to a magical school with robes and wands and not get in trouble. However if you took the additional steps of calling non-magical people Muggles, having a Ministry of Magic, implementing a sport called Quidditch and having many of the same classes and professor jobs that are in the Harry Potter franchise, then there is a good possibility that JK Rowling’s people will send you a strongly worded letter or even a lawsuit.
So, your world and your ideas are already protected to a certain extent as long as you publish them. If there are magical creatures that you have made up the names for that are in your books, then someone else cannot use the same creature way the you did. They may be able to use the same name as long as the creature is different. When it comes to common mythological animals, no one has any copyright claim over them. For example, if you include unicorns, manticores, Medusa and dragons in your work, the books that have already used those creatures are not going to be able to come after you and claim the you have violated their copyright. The same goes for your work. Now, that could be different if you’ve created an entire mega back story in the world of dragons and someone else uses that world to populate their own book with dragons. Even then though, you might be on shaky ground.
As for protecting your ideas that are not published or available somewhere, the best thing to do is to always upload your ideas and plots that you’ve created to some sort of Internet medium like Google drive. This allows you to prove that you had that party written with the date stamp on it. You should also avoid sharing really good ideas with other writers who may use them later on themselves even subconsciously.