Is fascism really an original 20th century phenomenon? Was there nothing functionally or ideologically similar to fascism at some point in history before the 1900s?

This is an interesting question that gets into intellectual history territory. I'm going to frame the question with some context to help you think about it better.

So in the late 1960s, Quentin Skinner published an article called Meaning and Understanding in the History of Ideas which revolutionized the field of intellectual history and provided the framework for the now-orthodox Cambridge School of Intellectual Thought. A large part of this article argued against the hitherto accepted methods for understanding the history of ideas:

Looking at solely what the text itself said
Looking at the context in which the text was written

Skinner argues that it is never sufficient to look at the text alone, nor its context, because then you run the risk of back-projecting modern ideas onto the past by starting from what we know in history. This is what's called a teleological fallacy and it's widely looked down upon in the field of history. Teleology is basically looking at what actually happened as we know it now, and then using that fact to understand the structural forces that got us there as if it was 'inevitable.' Think of it like reading history backward. One of the things you have to accept is that history is highly contingent, and what we know now may seem obvious, for example, the success of the German blitzkrieg, but at the time no one actually knew what was going on to happen.

What Skinner is cautioning against in critiquing these methods is the tendency for historians to read the past through modern terms or a modern lens. So when we apply this to say Fascism, there is and has been a very strong tendency to try and draw a line through ideologies from Early Modern Europe through to Fascism. We can take say Hobbe's Leviathan and look at that as the embryonic inception of a strong central state. However, Skinner argues, Hobbes, and all of these early thinkers, had absolutely no idea what Fascism was, and were merely responding to crises that transpired during their own time. Any attempt to project Fascism onto ideas before its time is, therefore, a construction of the subjective faculties of the historian, rather than something innate to the texts themselves. This is why it's important to take ideas within their historical context, and treat the historical development of ideology as contingent rather than predictable. Skinner thus argues for looking at what an author intended to influence with their writing within their time, and not just at the text or context alone.

However, the weird part about history is that it repeats itself, or 'rhymes' in Mark Twain's words. Now, the interesting question is, does this have more to do with the fact that there are discernible structural patterns in history, or, rather, that as people begin to be conscious of their own history they tend to repeat it either consciously or subconsciously? We know, for example, that people like Hitler and Mussolini were indeed drawing on a wide array of texts from the 1800s and Early Modern Europe. A decade after Skinner, Dominick LaCapra picks up on the tendency for history to influence the minds of historical actors, and writes Rethinking Intellectual History, in which he argues that the history of ideas is very much so alive in its readers, and it is therefore important to additionally look at how certain texts influenced their readers, and how certain texts were understood by their readers, even if that 'understanding' is divergent from the original author's intention.

Again, this has to do with the idea that we are all conscious of history. We grow up around it, and develop national histories and collective psyches based on what we learn about the past. Our understanding of history then influences us in the present, which then becomes history in the future. These are the complex layers and infinite facets to understanding the history of thought. Which is why, to answer your question, there is no easy answer.

I will add that there is an interesting book by Adorno and Horkheimer (two of the founders of Critical Theory) called Dialectic of Enlightenment which argues that Fascism is the zenith or highest form of the enlightenment--that fascism represents the over-rationalization of society, a path that it had been put on by the influence of enlightenment thought on political philosophy. I personally think that they have some good points and that the tendency to see Nazism as an 'irrational' regime that made self-destructive decisions had more to do with allied-driven historiography that painted them as madmen to 'rationalize' the violence and genocide. However, I subscribe to research like Adam Tooze's Wages of Destruction that posits Hitler's decision-making as rational with an eye to the macroeconomics of the Nazi War Machine. Still, to what extent this metamorphosis was inevitable I'm not sure.

So in the end, I guess the point I want to leave you with is that you have to be wary about trying to read the past backward, and you need to be conscious that the very nature of your inquiry falls easy prey to a teleological vision of the history of political thought that treats 20th century Fascism like a 'final form' of past historical developments. Part of the issue with this view is that we know Fascism was in part a reaction to WWI, and WWI for almost every year of the war, popular to contrary belief, was a highly contingent drama where either side almost won multiple times between 1914-1917 (until the industrial juggernaut kicked in on the allied side in 1918)--very different from the second world war where the material advantage by the end was just so great that many historians argue the Axis never actually had a fighting chance after 1941.

At the same time, there is something to be said of the progression of historical thought being in the eye of the beholder, and we certainly cannot deny that much of Fascist/Nazist thought was based on precedent. For example, much history has been done on the connections between Nazism and colonialism/manifest destiny; this is a whole other can of worms that I'm sure someone has made a good comment of in this sub before, so I'm not going to go into it here. The point is, many of the necessary conditions for Nazi thought (and also Fascist thought) were common to the Anglo-American and European worlds, and not 'new' phenomena. Racism, for example, certainly was not invented by the Nazis. However, this does not mean that early modern thinkers had something even akin to fascism in mind when they were formulating their own theories or philosophies about the world. Many of them long dead might not even have consented to their ideas being appropriated by Nazism (for example Ernts Junger was not a Nazi supporter despite them appropriating some of his work), and we also have to be wary of the ways in which Fascism and Nazism itself misread its own history in deliberately intellectually dishonest ways, most famously seen in the way that Hitler misappropriated the legacy of Frederick the Great.

To conclude, I think the essence of your question comes down to accepting the full logical extremity of paradigm shifts between epochs in history. Michel Foucault in The History of Madness and the Order of Things makes a case for the very idea of reason being contingent on time period. He demarcates roughly the 1600s to the modern period into three separate eras (Renaissance, Classical, Modern), and argues that each epoch has epistemes (preconditions to knowledge) that were completely different. He is, in effect, arguing that the way peoples brains were wired, and their understanding of reality (which you can make a case for reality itself) was completely different and perhaps unintelligible to other epochs. So when we look at the pre-modern period for example (before 1800s), even though it is tempting to draw connecting dots through these ideas that do look very similar at a glance, and supported by the fact that we know Fascist thinkers were themselves cognizant of, we still have to recognize that the pre-1900 world was a radically different paradigm, a whole separate world one might say, in which these thinkers were conceiving of the early ideas that we like to causally attribute to Fascism/Nazism in a different context. We need to take seriously both their merits as independent ideas formed under different circumstances, as well as their propensity for feeding these destructive 20th-century ideologies.

Infinite dialogues can and have been written on this topic but I hope this helps you conceptualize your question better.