Some more medieval examples of imposters are listed on page 36 of The Atheist’s Bible: The Most Dangerous Book That Never Existed, Georges Minois, 2012, is the statement that in the period 1300 to 1500:
Impostors were everywhere: A false Baldwin IX, a false Alfonso I, a false Fredrick II, a false Henry V, a flase Conradin, a false Edward II, a false Richard II, a false Valdemar II, a false Warwick, a false York, a false Joan of Arc, false Popes, and even a false female Pope Joan.
I may add that there may have been two imposters claiming to be King Olaf )1370-1387) II of Denmark and IV of Norway, one in 1402 and one in 1387 or 88.
The one in 1402 is quite well known:
http://www.executedtoday.com/2008/09/28/1402-false-olaf-iv-michaelmas/[2]
The false Olaf from 1387 or 1388 is mentioned in The Middle Ages: Dictionary of World Biography, volume two, 1998, page 627:
“In Norway, a disaffected faction reported that Olaf was not dead. The Impostor claimed the crown and gained followers by revealing information that only Olaf or Margaret could know. Margaret hurried to Norway and proved that the impostor was the son of Olaf’s nurse by showing that he did not have large wart on his back Olaf’s birthmark. The false Olaf was tortured and burned at the stake.”
https://books.google.com/books?id=aBHSc2hTfeUC&pg=PA627&lpg=PA627&dq=margaret%2Bof%2Bnorway%2Bimposter%2Bburned%2Bat%2Bthe%2Bstake&source=bl&ots=qNCqy6KGbn&sig=ACfU3U3MoaOQKtFTpuLePz_XRJlG6CHuZg&hl=en&ppis=_c&sa=X&ved=2ahUKEwjc3IyAm8PnAhUsuVkKHfdUDtMQ6AEwD3oECA8QAQ#v=onepage&q=margaret%2Bof%2Bnorway%2Bimposter%2Bburned%2Bat%2Bthe%2Bstake&f=false[3]