"The Boarwolf" is a great story, and also a bit of a puzzle. Everett Bleiler attributed this early tale of lycanthropy (and porcanthropy?) to Johann August Apel, in the Tales of Terror entry of his 1983 Guide to Supernatural Fiction (p. 441). This attribution persists; it's repeated in the Internet Speculative Fiction Database. And the 1997 Encyclopedia of Fantasy not only attributes the story to Apel, it further claims a publication date of 1812.
However....
The text of the story as given in Tales of Terror (and later in Evening Tales for the Winter) comes from Tales of a Voyager to the Arctic Ocean, Vol III (1826), by Robert Pearse Gillies. In that novel (which is also the source of "The Nikkur Holl"), "The Boarwolf" is a story told by the sailor Maerts Duytkin to his companions. The chapter is entirely in quotes, which were preserved when Henry St. Clair reprinted the story in his anthologies--though he, or whichever source St. Clair took the story from, Americanized some of the spellings.
In other words, the story is presented as being original to R. P. Gillies. Gillies was a noted critic and translator of German literature, and it's entirely possible the "The Boarwolf" is his translation or retelling of an originally German tale. It certainly feels like a German Gothic tale: the main characters are two young hunters from the Bergstrasse, and the narrative contains themes not only of lycanthropy, but also the ever-popular "deal with the devil goes awry" motif. This makes the story rather more literary than folkloric, in my opinion, since traditional werewolves are more closely associated with vampires than with the Devil.
But if the original story is by Apel, what is it? The 1812 date cited by the Encyclopedia of Fantasy puts it right in the midst of the publication of Gespensterbuch (Ghost Book), but I haven't found anything from that collection that looks similar. Of course, this is based on looking at the titles of the stories on German Wikipedia, and some searches on Projekt Gutenberg-DE, so who knows.
If any experts on German Gothic literature, or on R.P. Gillies, out there can verify the Apel attribution (or provide another attribution) for this story, I would love to hear about it. But until then, I think I have to credit Gillies as the author of this piece.
The first link in the article above goes to a clean PDF of my transcription of "The Boarwolf."