u/BaffledPlato last week provided a great example about a man named Alexander of Abonoteichus in the second century CE, who set up a scam religious cult.

Another one is the story of a Mede called Gaumata, who held the Achaemenid Persian throne for a while in the 6th century BCE by pretending to be a member of the ruling dynasty. Maybe somebody with a better knowledge of Persian history can provide details how this story is interpreted today.