"Reptilicus" has been called "Denmark's only giant monster film", but that's not quite correct. There's also a giant monster in Ib Melchior & Sidney W. Pink's Journey to the Seventh Planet (1962), filmed in Copenhagen back to back with "Reptilicus", as well as in Henrik Mikkelsen & Peter Nielsen's monster shorts "King Kong '86" (1986), "The Great Pink" (1986), "Fortidens ø" (1986) and "Rædsler på fortidens ø" (1986), all four filmed in Randers, Denmark.
A paperback tie-in novelization of this film, written by Dean Owen (real name: Dudley Dean McGaughy) and published at the time of its release, is notable for its inclusion of gratuitous sexual scenarios that were entirely unrelated to the film. The film's producer/U.S. director, Sidney W. Pink, was so enraged by this that he sued the company that published the novel, Monarch Books.
Two versions of this film were made: a Danish-language version directed by Poul Bang and an English-language version directed by the film's producer, Sidney W. Pink. All of the actors in the film were in both versions, except for Bodil Miller, who could not speak English; as a result of this, Marlies Behrens dubbed her character. The head of American International Pictures, Samuel Z. Arkoff, said that the dialogue had to be re-dubbed because he felt that the "sing-song Scandinavian accents" would have U.S. audiences laughing. At first, Pink threatened to sue AIP. However, after Pink had other industry professionals watch his version of the film the lawsuit was dropped and AIP released an edited and re-dubbed English-language version of it after all.
The film's producer/U.S. director, Sidney W. Pink, wanted to make a sequel to it in 2001, which would have taken place 60 years later, in order to cash in on the success of the film Godzilla (1998). Unfortunately, Pink died in 2002 before pre-production on the sequel could even begin and it was never made.
A few scenes in the Danish version of the film in which Reptilicus was shown flying were cut from the U.S. version of it before it even reached the U.S. because its producer/U.S. director, Sidney W. Pink, thought that they looked "unconvincing".