The rank of Knight has exactly one holder in the whole catalogue. Furcas, the fiftieth spirit, commands twenty legions and is the only spirit of the seventy-two to whom the Ars Goetia gives the title.
A category with one member in a working taxonomy is nearly always a sign of something. Either it is a residue, a class inherited from an older source that the compiler kept without needing, or it is a deliberate outlier put in to mark the bottom of a scale. Both readings are available here and neither can be confirmed from the text, but the structure gives a hint. The other six ranks descend in an orderly way, and the rung below Earl in a human peerage is not knight at all: knighthood is a personal dignity rather than a peerage rank, and it sits outside the ladder rather than beneath it. Reaching for it once and never again is what a compiler does when the borrowed ladder has run out and one more entry still needs placing.
Furcas himself is not a small figure. He is described as appearing in the form of a cruel old man with a long beard, riding a pale horse and carrying a sharp weapon, and he is credited with teaching philosophy, astronomy, rhetoric, logic, chiromancy and pyromancy. That is a longer list of subjects than most of the Presidents are given, and it sits oddly beneath a rank that ought to be the humblest in the book.
Rank and function, once again, do not predict each other in this catalogue.
Sources Lemegeton Clavicula Salomonis (The Lesser Key of Solomon)Anonymous (unidentified compiler) with Encyclopaedia Britannica (online academic reference)Encyclopaedia Britannica editors