Well, the wording of your question really strikes at the heart of what makes this question actually so difficult to answer. Leaving aside the question of ‘prophet’ (which is often just a class of official priesthood, particularly associated with oracles), you ask about there being ‘religious reformers’ in various pagan religions. But to reform something, it needs to have some form in the first place – it has to have been formalised so that a reformer can reformalise it into something new. I can’t speak for Germanic, Celtic, and Slavic religions, but Greek, Roman, and Egyptian religions never really had any consistent ‘form’, and instead had a great deal of multiplicity. The huge geographical spaces that made up the Greek and Egyptian worlds meant that any formalised religion was essentially impossible, and although basic frameworks existed people tended to worship local and deeply personal gods in distinct ways. Apollo might have broadly been one god on a pan-Hellenic scale, but the Apollo worshipped at Delos and the Apollo worshipped at Cyrene were also fundamentally quite different and distinct divinities. Similarly, the Egyptians had no settled creator god, and different cities had their own local cosmogonies: was the world moulded out of clay on the potter’s wheel (Khnum, at Elephantine), or spoken into being (Ptah, at Memphis), or created through a god masturbating into his own mouth (Atum, at Heliopolis)? There was no ‘formal’, well-defined Greek/Roman/Egyptian religion: it was essentially a hodge-podge of different cults and customs, which often contradicted each other but never rendered each other invalid. This leads to a further issue, which is that all of these religions were highly civic: priesthoods could be public offices, and any festival would involve the entire city participating. The local, civic nature of these religions means that any ‘reform’ would be generally internal (rather than the highly external focus of e.g. Christianity): any reform of Greek religion could only happen through similar reform of Greek society, and this would be catastrophic. And that level of internal, violent social upheaval is fundamentally not sustainable, so any major ‘reform’ is highly unlikely to last.
Nevertheless, there are a few examples of individuals making significant, rapid effects on religion, so it’s worth considering them. Though Egypt’s religion was primarily local in focus, the king was pan-Egyptian and therefore provided a focus for Egyptian religion throughout the country. In the 18th Dynasty, the kings had originated in Thebes, a huge city far to Egypt’s south, and put an unusual amount of effort into essentially nationalising Thebes’ chief god Amun, to some extent making him supplant Re (the traditional sun-god) as the chief pan-Egyptian god. So whereas in the Middle Kingdom we find kings narrating their conception with Re as the father, to emphasise their divine legitimacy to rule, in the New Kingdom Amun now becomes the king’s father. Unfortunately for later members of the 18th Dynasty, the royal centre of power was traditionally around Memphis, in the north of Egypt, while Thebes was mostly run by the hereditary priesthood of Amun. This created a crisis in determining legitimacy: the king derived legitimacy from Amun/Thebes, but the city/temple was now sufficiently powerful that this could not be reliably controlled. Amenhotep IV therefore essentially abolished Amun, removing all of the priesthood’s power and carving out the god’s name from any monument. Instead he promoted worship of the Aten (the Egyptian word for the Sun’s disc), and emphasised his own unique ability to communicate with this Aten, for good measure also renaming himself Akhenaten and building a city (Akhetaten) in roughly the geographical centre of the country, so that it was easier to exercise political and religious control over the south. He didn’t make Egypt monotheistic: all other gods were viewed as manifestations of the Aten, but this idea is pretty typical for Egyptian religion – Akhenaten just puts more influence on the one (the Aten) than the many (the other gods), but they still were very much believed to exist. At any rate, as noted above, this religious change necessarily entailed huge societal change as well, with the king and his family more centralised in the social hierarchy than ever; when Akhenaten died relatively young, the resulting power vacuum allowed the decentralisation of society and religion to happen very, very quickly.
The geopolitical tensions did not go away after this, however, and so Amun is also marginalised during the reign of Ramesses II, though to a much lesser degree. In later inscriptions from the king’s reign, we find him being crowned not by Amun but by a deified version of himself, a physical manifestation of the king’s divinity. Ramesses II therefore derives his legitimacy from a version of himself and does not need to rely on Amun and the priests who control him. This reform was too minor to really achieve much, though – it ensured Ramesses II’s and his heir’s legitimacy, but the priesthood continued to accrue power and eventually became de facto independent during the 21st Dynasty.
We also find at least one example of a ‘reformer’ of Roman religion after Christianity had begun to spread. In the small city of Abonoteichos on the north sea of Turkey, around 130 AD, a young trainee magician named Alexander hatched a snake and proclaimed this to be Glycon, the son of Asclepius (and so grandson of Apollo). He then set up an oracle inside his house, got a large, docile, pre-bought snake to wrap itself around him a bit, and then put on a hand-puppet of a snake/human-head with human hair (there’s a lovely marble statue of the god that was found in Romania, which you should be able to see online). He then proceeded to give prophecies by asking devotees to write and seal their questions on scrolls and bury them; he would then unseal these during the night, read them, reseal them, and provide a perfect answer. Despite essentially being a sock-puppet, Glycon became hugely popular, especially because a particularly superstitious Roman named P. Mummius Sisenna Rutilianus was tricked into marrying Alexander’s daughter. Rutilianus became highly influential in the empire, and so when Alexander prophesied that Marcus Aurelius should throw two lions into the Danube to guarantee victory in war, he actually seems to have done this (the Romans in fact lost, and so Alexander used the classic oracular defence that he hadn’t said who was guaranteed victory). Even after this, however, the cult of Glycon continued to grow in popularity: coins had been minted in honour of Glycon under Antoninus Pius (Marcus Aurelius’ predecessor) and continued to be used into the 3rd century AD. Obviously, this cult eventually faded in popularity, but it’s a fascinating pagan parallel to Jesus, showing how charismatic individuals could have massive effects on religion across the Roman Mediterranean, no matter their belief system.
Further Reading
Bickel, S. (2002), ‘Aspects et fonctions de la déification d’Amenhotep III’, BIFAO 102: 63-90
Frood, E. (2019), ‘When Statues Speak about Themselves’, in Masson-Berghoff, A. (ed.) (2019), Statues in Context: Production, Meaning and (Re)uses (Leuven): 1-18
Habachi, L. (1969), Features of the deification of Ramesses II (Glückstadt)
Laboury, D. (2020), ‘Aten vs Amun: Religious Politics and Political Religion under Tutankhamun and his Father, Akhenaten’, in Connor, S. & Laboury, D. (eds.) (2020), Tutankhamun: Discovering the Forgotten Pharaoh (Liège): 238-42
Lurson, B. (2016), A Perfect King: Aspects of Ancient Egyptian Royal Ideology of the New Kingdom (Paris)
Redford, D. B. (1976), ‘The Sun-Disc in Akhenaten’s Program: Its Worship and Antecedents’, JARCE 13: 47-61
Thonemann, P. (2021), Alexander, or The False Prophet (Oxford)