Ray Harryhausen used stop-motion animation to create the various creatures in the film, including Calibos, his vulture, Pegasus, Bubo the mechanical owl, Dioskilos, Medusa, the scorpions and the Kraken. Harryhausen was also co-producer of the film, and retired from film-making shortly after it was released. Despite Bubo's similarities to the droid R2-D2 of the 1977 film "Star Wars", Harryhausen claimed Bubo was created before "Star Wars" was released. The BBFC, reviewing the film for certification in 1981, said Harryhausen's effects were well done and would give entertainment to audiences of all ages, but might appear a little "old hat" to those familiar with "Star Wars" and "Superman".