This film was initially released in America in a 2D version. That version of the movie was pulled during its theatrical run despite the fact that it was doing good business and was replaced by a 3D version that flopped at the box office.
The original intention was to set the story in Spain and be about a Galician or Asturian werewolf, but the Spanish censorship of the era would not allow such a story to be set in Spain, or such a character to be a Spaniard, so the movie is set in a Teutonic country, and a Polish werewolf was created.
Jacinto Molina's debut as screenwriter. He was convinced to star as well, thanks to the German backers who salvaged the production. Forced to choose a non-Spanish acting name for the international market, he picked 'Naschy' from a friend, Imre Nagy, while 'Paul' was inspired by the then-current Pope, Paul VI.
When Samuel M.Sherman was screening foreign titles for a suitable 'Frankenstein,' he actually passed on "Assignment Terror" (original title "The Man Who Came from Ummo"), Paul Naschy's sequel to "La Marca del Hombre Lobo," which was completed in 1967, two years before its followup. Ironically, the film he passed on combined Naschy's werewolf with a Dracula, Mummy, and Frankenstein monster, though under different names, and starred Michael Rennie and Karin Dor, two internationally known actors.
Samuel M. Sherman had promised distributors a film called Blood of Frankenstein but could not deliver on time. This would eventually become Dracula vs. Frankenstein (1971). To satisfy the distributors he found La marca del Hombre-lobo (1968) and retitled it Frankenstein's Bloody Terror.
The US print distributed by Samuel M.Sherman was retitled "Frankenstein's Bloody Terror" because of pre-arranged bookings promising a Frankenstein feature. The director's name was changed to 'Henry L.Egan.' Only six cast members were listed on screen- 'Paul Naschy,' 'Diana Zura' (Dianik Zurakowska), 'Michael Manza' (Manuel Manzeneque), 'Anita Avery' (Aurora De Alba), 'Rosemarie Winters' (Rossana Yanni), and 'Gilbert Granger' (Gualberto Galban). There were no other credits on screen, so US audiences were denied any knowledge of Paul Naschy's real name, Jacinto Molina, which he would use as both writer and director in the following years.