Was National Socialism, the ideology of the Third Reich, invented by Hitler or did it exist before, possibly under a different name?
Nazism may be described as a version of fascism that incorporates antisemitism and the so-called scientific racism. This particular package of the themes was new and invented largely by Hitler (although there were probably many, less famous or infamous people whose beliefs corresponded to the very same mixture). However, the individual pieces were not new at all.
Fascism is a radical authoritarian nationalism, energized by its opposition to liberalism, anarchism, and Marxism. The modern nationalist roots go back to the 19th century when nation states of the modern type started to be born while the particular opposition to the other ideologies only arose after those were established, e.g. after Marx's ideas were converted to practice.
Hitler was not the first fascist. The first successful fascist party was Benito Mussolini's party founded in 1919.
Antisemitism has existed in the Christian Europe for a very long time, basically from the era of Jesus Christ who was "crucified by the Jews" (those who say this thing usually overlook that Jesus was another Jew, too). Many countries experienced a strong wave of antisemitism in the late 19th century and the term was coined in 1879.
Scientific racism has been debated since the enlightenment, around 1650, and lots of great thinkers were involved in the debate in one way or another. In the early 20th century, eugenics and similar ideas were extremely popular among many academics. They looked innocent because they seemed largely detached from the real-world politics. It may be accurate to say that Hitler was the first one who brought scientific racism and especially its hostile form to the Realpolitik, who combined the previously scholarly dangerous ideas with the ordinary people's emotions and their desire to act.
Aside from these pillars of the ideology, Nazism may also be said to include many other things – perhaps also positive things – associated either with the German approach to many affairs in the society and the economy or Hitler's idiosyncrasies. Nazism was also inspired by some old traditions of the German nation as well as some philosophers such as Nietzsche. It was really a strange package of assorted ideas, beliefs, plans, and emotions.