"Historicus" was a psuedonym for a writer who remains unidentified. He/she wrote a column in the Anti-Polygamy Standard entitled “Sketches from the History of Polygamy, The First Polygamous Marriages.” (See 10 [January 1881] 10: 1 and 2 [April, 1881] 1: 1.)
From the claims made in the columns, it appears that he was not a contemporary of Nauvoo polygamy, but was an author who performed research and drew his own conclusions. Based upon the information he provided, one of his informants may have been William McLellin.
Latecomer accusers are only as reliable as their historical sources, and as a columnist for the Anti-Polygamy Standard, objectivity would not normally be expected. How extensively his biases influenced his interpretations is a question left to readers.
It appears that "Historicus" was the very first author to publish Fanny Alger's full name as "Fanny Olger or Alger."