There's no shortage of excellent responses to prior questions about fascism in this sub, which I hope somebody will link as well. There were some antecedents in Italian politics. Garibaldi and Mazzini were highly regarded by many fascists, but I think it makes more sense to view them as characterizing the variety of Italian leftism with which Mussolini initially broke than with where he arrived. Both advocated popular sovereignty under a democratic republic, and both (especially Garibaldi) were anti-colonial. Francesco Crispi was perhaps Mussolini's clearest domestic predecessor.

For reasons which have been better explained in those prior answers, fascism is quite a difficult political idiom to pin down. The most popular definition right now is Roger Griffin's, which considers the defining characteristic of fascism to be the way in which it constructs a political narrative in order to justify "national rebirth" through ethno-nationalist revolution. Implicit in this is are some incredibly self-contradictory assertions about the history and character of the nation, however it is defined by the fascist in question. There must be, according to this definition, a moment at which the nation was held to suffer humiliation at the hands of an inferior other, as well as a moment of triumph which can be regained through the revolutionary rekindling of the national conscience under an authoritarian leader.

The thing is, this makes fascism, in many ways, an inherently modern ideology. It could not have existed without the social innovations which allowed nationalism itself to take root, or those which facilitated the sustainable rise of mass politics, like widespread literacy, mass communications, and industrialization. Early Italian fascists were all hopped up on futurism, a cultural and artistic movement which idealized the potential of industrial technology to complete humanity's mastery over nature. Fascism's narrative of domination through the unification of workers, peasants, and capitalists into a single national "machine" appealed to many of the futurists, and Mussolini's regime (at least early on) embraced many of the futurists, before the more revolutionary tendencies were purged from the Fascist Party as the movement coalesced around a more reactionary character.

Still, that portion of fascism not dependent upon modern mass politics--the heavily reactionary, nationalistic, and authoritarian core of it--has plenty of antecedents. I believe that the clearest antecedent is probably the Imperial Russian state ideology known as Official Nationality, the three tenets of which were declared by Tsar Nicholas I to be "Orthodoxy, autocracy, and nationality." Political, religious, and social control was to be centralized in the person of the Tsar, the single leader of the Russian people, and by extension all Slavic peoples. This ideology formulated around a national narrative of humiliation--the subjugation of the East Slavs under the Golden Horde--contrasted with the triumph of liberation from the Golden Horde, and the success of Russia's conquest of Siberia. It is difficult to overstate how large the threat of attrition along Russia land borders with Central and East Asia loomed in the collective political psyche of Imperial Russia. The narrative was justified with assertions of the racial superiority of Orthodox Russians, who supposedly embodied the best of both Europe and Asia, while also claiming the legacy of Imperial Rome through their position as the dominant Orthodox Christian state. This ideology was used to justify the brutal persecution of non-Slavs and non-Christians, especially Jews, many Muslims (though Tatars were relatively tolerated) and indigenous populations in Siberia, the Caucasus, and European Russia, where Uralic and Turkic speaking ethnic minorities were forcibly Christianized to varying degrees of success.

Imperial Russia also adopted some elements of the modernist narrative of mastery over nature through a synthesis of industrial progress and ethnic superiority, especially during the reign of Nicholas II, who had an almost obsessive fixation on the Trans-Siberian Railway, a project which many advisors saw as an ideological boondoggle with very little economic utility. Still, with Nicholas's blessing, the Russian Orthodox Church led the recruitment and resettlement of Slavic families in Siberia and the Far East, even going as far as sending a delegation to recruit Polish and Czech Americans in Chicago, which failed spectacularly.

The Russian nationalist incarnation of pan-Slavism is a pretty clear precursor to many fascist racial ideologies. Slavs were the superior race, and ethnic Russians were the natural leaders among the Slavic peoples. Ukrainians, Belarusians, Poles, Serbs, etc. were welcome in the ranks of the master race, so long as they were willing to emulate proper Russian ways and relegate any collective identities to a subservient place. The Slavs, of course, were naturally inclined to expand to the East, where they would settle and subjugate the local populations. (Aside: the settlement of Siberia has striking similarities to the Anglo settlement of North America, and especially the West, which the Russians were acutely aware of.) So, encompassed in Official Nationality, we have something like Aryanism, and something like lebensraum.

All of this was enforced by a series of secret police organizations with antecedents as far back as the 1600s, but best exemplified by the Okhrana, which was set up in 1866 (the same year that serfdom ended) to suppress separatist and leftist agitation.

Where Russian imperialism arguably deviates is in its use of something more akin to "permanent reaction" than national revolution. As long as the Tsar reigned, the nation would prevail, but permanent vigilance enforced by a strong state was seen as necessary to suppress threats to the regime. In this sense, Official Nationality claimed the ideal which fascists profess to pursue, and pushed a narrative of imminent national death, with glory possible only through a whole-nation defense of the tsarist order.