Anglo-Saxon mythology
J.R.R. Tolkien's The Lord of the Rings was influenced by the myths of the Northern Europeans. As it became popular, parts of its fantasy world moved into how people see the fantasy genre. In almost any modern fantasy novel, you can find Germanic creatures like elves, dwarves, and giants. Tolkien was also strongly influenced by his experiences fighting during the First World War and created his own mythology, drawing heavily from Anglo Saxon myths as well as Welsh. Thus he wrote the books in which he created over 30,000 years of history within The Simarillion, of which the entire Lord of the Rings storyline only takes up a small part of.