I have been told that there are Pirushim on the Torah who speak of werewolves.
Can anyone locate them?
The (rarely-seen) commentary of Rabbeinu Ephraim ben Shimshon on the Torah states that Benjamin was a werewolf; the fear that he would die should he leave his father Jacob is that he would turn back into a werewolf and be killed in self-defense by some person. Make of it what you will. Hat tip to Yitzhak of Bein Din L'Din. (See link for more.)
Despite this being an old question, it recently came up in conversation, so I'll take a stab at it. Three of these sources (the exception being the Chizkuni, which I found myself) come from a footnote to Eliezer Brodt's article on ברכות הראייה printed in Yeshurun vol. 26
There are indeed a few commentaries that mention werewolves:
Rashi, in his commentary to Iyov 5:23, explains that חית השדה there means 'gerusah', which, according to this scholarly blog post quoting the Otzar Haloazim no. 4208 means 'werewolf'. Eliezer Brodt, however, believed that this is a misprint in Rashi and in fact might contradict what he writes elsewhere. (I myself find this translation hard to believe anyway, due to Rashi's use of this word in Koheles 12:11 to mean 'round heads')
Chizkuni to Beraishis 16:12 explains that Yishmael was destined to be a פרא אדם, which, according to one explanation, is a werewolf (at least according to the footnote in Moshe Menachem Aharon's edition, pg 88)
R. Menachem Tzioni, at the end of Parshas Noach, writes that the men of the dor haflagah (those who participated in the Tower of Babel) were punished by being turned into werewolves, among other things
R. Yehudah HaChassid, in Sefer Chassidim no. 464 (pg. 318 in Margolios edition), also mentions werewolves (and dragons, etc.) and how to avoid them
Rabbeinu Efraim, in his commentary on the Torah (49:27), compares Binyamin to a werewolf, and he also discusses the creation of werewolves in his comments to Beraishis 2:28. In his comment to Beriashis 44:29, Rabbienu Efraim actually posits, according to one interpretation, that Binyamin actually was a werewolf, and this fairly shocking claim is noted by this blog author, as well as by R. Slifkin, here.
R. Moshe Taku in the fragment we have of Ksav Tamim apparently mentions werewolves and their category in creation, though I haven't yet looked at this source myself
Rabbi Yehuda Hachasid talks about a werewolf and another type of being.
There is a Christian belief that at the end of Nevuchadnetzar's life he turned into a werewolf, however I have never seen any Jewish Perush saying so.