Who is the pioneer of populism? Hitler is turning in his grave right now.
I'd argue Hitler was not really populist in the conventional sense. He (and his party) was firmly rejected for a good decade or so by the German public (and trust me they tried their darndest to be popular with them both conventionally and unconventionally)....their core message was the same back then as it was later.
What changed is they were able to infiltrate, coerce and use violent mobs on political opponents (too often the focus is only on the brownshirt vs reds....but it was far far more than that, those were just the most brutal open-air fights visibly...ironically over the same kind of extremist thinking base of ppl at large if you look at how factions somewhat seamlessly switched).
One of many examples was the final German parliamentarian (of SPD I think - party that still exists today and come to power under Shroeder etc...but I forgot the guys name) that was doggedly against Hitler/NSDAP but he was coerced in some mysterious way to relent and vote for mandate hindenburg was trying to establish in name of stability (this is a long subject to get into)...given the 2nd round of economic calamity from the depression (largely originated in the US and fuelled by debt-recall in foreign countries like Germany)
It was all mostly reactionary driven and it changed to pseudo-populism to some level only afterwards (1932 afterwards I mean), when the actual levers of economic policy, institutions and mass media could be totally subverted and then utilised to be made to drive a more traditional populist front (mostly to help disguise the earlier root agenda/goals). But really it was not populist in its genesis and larger approach at all....neither social populism or economic populism. Elements it took on of that, well there is no easy way to reference the genuine popularity among the people as the means to measure that were all controlled/subverted at that point by an extreme authoritarianism.
This is also what allows for German public at large to not be suddenly radicalised to such extreme level in just a few short years....and then magically flipped off/reset again in 1945.
Genuine root populists (given a democracy need exist in first place for this term) you would have to do an analysis of 19th Century US and UK....whigs vs 19th century dems in the US (as to what was even populist back then compared to now) and also different kind of whigs found in the UK vs tories vs liberals.
In 20th century non-democratic authoritarians/totalitarians, I would actually say Mao was the largest innovator overall of populism (he saw and harnessed what the peasants numbers and desires were) and Lenin to some degree too....as the institutions + reverse inertia (against populism) were respectively not well-established and low in societal energy....so populism actually had a huge ripe sweetspot condition to flourish in first place (often called a revolution), unlike the case with Hitler/NSDAP as they had to work their way into existing power corridors in a very anti-populist (few ppl involved) method.
Sorry to go off topic a bunch....it was to also help air out some matters as they rest in my head on this topic.