The prehistoric sea monster that became the Rhedosaurus was initially conceived by the writer Ray Bradbury for his short story "The Fog Horn", which appeared in the June 23, 1951 issue of The Saturday Evening Post.

Prior to deciding to adapt Bradbury's creature from the story and the artwork by James R. Bingham, Ray Harryhausen, and Eugène Lourié went through many draft designs for producers Hal E. Chester and Jack Dietz, who desired to make a monster film due to the successful 1952 re-release of King Kong (1933).

After considering using existing dinosaurs such as an Allosaurus, Harryhausen and Lourié eventually decided to invent a new fictional creature; its appearance later infuriated scientists and students alike upon its release.

Harryhausen ultimately made two models based on his concept art before concluding on a fearsome-looking design with the producers' approval and portraying the creature in the film via stop motion animation.

[4] In the film, the dinosaur is awakened from its over-100-million-year slumber in suspended animation under the Arctic by an atomic bomb detonation and travels south towards the location of its original home, now New York City, on a devastating rampage.

[6] In that film, it inhabited a Mesozoic era Earth analog which a human crew becomes stranded; this time, however, it is a smaller, brown dinosaur that is easily defeated by a larger Tyrannosaurus that chomped into his head, seemingly killing him.

[10] The prehistoric sea monster that became the Rhedosaurus was originally conceived by the writer Ray Bradbury for his short story "The Fog Horn", which appeared in the June 23, 1951 issue of The Saturday Evening Post.

Bradbury's friend Ray Harryhausen was also assigned to work on Chester and Dietz's film by this point and was given a copy of James R. Bingham's artwork of the creature published alongside the story in The Saturday Evening Post.

[3] Following a few rough sketches of the beast's structure on paper and discussing storyboards for the film, Ray Harryhausen constructed a clay prototype of the Rhedosaurus and then used it to make a model out of latex, which he revealed to the two producers.

In his book The Art of Ray Harryhausen, he stated that it gave the beast what he desired was a more scarify look, however, after making some test footage he "realized that it still wasn't right" and remade it a third and final time before using it for the film.

[12] To portray his stop motion animated model of the giant beast tumulting in a particular setting in the 1953 film, Harryhausen invented a method for screen projection that divided the plates into foreground and backdrop imagery one frame at a time.

[7] In its debut film, the Rhedosaurus is depicted as able to easily adaptable to different environments, having survived in suspended animation for over 100 million years, and equally capable of moving on land and swimming under the ocean.

[16] In an attempt to placate them, director Eugène Lourié told reporters they invented the creature because the crew felt an unoriginal creation would not meet their intention of striking fear into twentieth century theatergoers:[11] "We wanted a brand new monster who looks more frightening enough to throw a large city into a panic.