The character Calibos, Lord of the Marsh and son of Thetis, does not appear in Greek mythology, and is based on Caliban, an antagonist created by William Shakespeare in 1611 for his play "The Tempest". In Greek mythology the son of Thetis was Achilles, Greece's best warrior in the fight against Troy.
Bubo, the mechanical owl of Athena, was introduced to capitalize on the popularity of R2-D2 from Star Wars: Episode IV - A New Hope (1977). The name "Bubo" is a scientific term for the genus of eagle owls and horned owls, which is interesting because the robot Bubo is modeled on a barn owl, which is the genus Tyto, and not a Bubo at all. Bubo makes a cameo appearance in this movie's remake Clash of the Titans (2010).
The Titans were the gods who preceded the Olympians in power. Kronos (also spelled Cronus) and Atlas were the most famous Titans. Ironically, none of the Titans from Greek mythology appear in Clash of the Titans (1981). In the movie the Titans are the Norse Kraken (who never appeared in Greek mythology at all) and Medusa (who was never considered a Titan by the Greeks).
Screenwriter Beverley Cross was a student of mythology. He developed a storyline centering on Perseus and Andromeda which linked together a number of myths. He took the idea to producers Charles H. Schneer and Ray Harryhausen. The story was modified to add more creatures.
The sea monster seen at the start of the film that destroys the city of Argos was derived from Norwegian mythology. In Greek, the sea monster that threatened Andromeda's people was called Cetus (whale). The Norwegian/Swedish name Kraken is now used as a synonym for the giant squid.
According to Harry Hamlin, prior to choosing to do the film, he was considering doing another project with distinguished actor Richard Burton, a film adaptation of Tristan & Isolde. He ultimately gravitated towards what became Clash Of The Titans despite not knowing anything about it, as it would have been an opportunity to work with Laurence Olivier, the one actor he considered probably even greater in stature than Burton. Tristan + Isolde (2006) ultimately wouldn't be brought to the screen until 2006.
In an interview with 'Cinefantastique' magazine, scriptwriter Beverley Cross said: "I had the idea for 'Clash of the Titans' in 1969 while I was living in Greece, on an island called Skiathos. It's very close to Seriphos, the island where legend has it that Perseus, the son of Zeus, was washed ashore in a trunk."
The original script called for Perseus to cut off Medusa's head simply by throwing his shield at her, in an attempt to appease UK Standards and Practices censors (as the producers felt that the hero decapitating someone wouldn't be appropriate for children in the audience). Harry Hamlin was apparently resistant to the idea from the beginning, as it wasn't in keeping with the actual Greek Mythology. When the day came to film the scene and it still hadn't been changed, he threatened to quit the film and fly home, and remained in his trailer, much to the producer, director, and Ray Harryhausen's annoyance. In the process of trying to coax him out, he was gradually able to get some of the other crew members on his side, which resulted in the scene being rewritten accordingly.
Greek God and Goddess characters appearing in the film include immortals Zeus (the Supreme Father of Gods & Men); Hera (Goddess of Marriage and Maternity); Thetis (Goddess of the Sea); Aphrodite (Goddess of Love); Poseidon (God of the Sea); Hephaestus (God of Fire, Metalworking, Stone Masonry, and the Art of Sculpture) and Athena (Goddess of Wisdom, Warfare, Divine Intelligence, Architecture and Crafts).