While visiting his friend Ray Harryhausen on the set, Ray Bradbury was given a copy of the script (which was going under the working title "Monster From the Sea") and was asked if he could possibly do some rewriting on it. After reading the script, Bradbury remarked about a scene in the story (which featured the monster destroying a lighthouse) that seemed very similar to a short story that he had published in "The Saturday Evening Post" several years earlier called "The Beast from 20,000 Fathoms". Bradbury's story was about a dinosaur that destroys a lighthouse. The next day Bradbury received a telegram offering to buy the film rights to the story. After the sale, the film's title was changed to "The Beast from 20,000 Fathoms". When Bradbury's story was reprinted years later, he changed its title to "The Fog Horn".
Deleted Scene: The 2003 DVD release reveals one shot of the Rhedasaurus that was omitted from the final film. That shot can be found in the trailer for The Black Scorpion (1957) (in special features) about 1/2 through the preview. (Spoiler: The Beast is walking, breast high, toward screen right. The background shows 2 buildings; one of them with fire escapes. Superimposed title card states, "You've thrilled to the terror of The Beast From 20,000 Fathoms."
Some film aficionados might recognize Alvin Greenman, the first character to speak after the narrator, and the first to notice the beast on on the radar. Six years earlier he played Alfred, the Macys Janitor in Miracle on 34th Street (1947). TV aficionados though might recognize the second character to speak. Playing the part of Charlie is actor James Best, best remembered for his role as Sheriff Rosco P. Coltrane from The Dukes of Hazzard (1979).
Before the film was sold to Warner Brothers, it contained an original music score composed by Michel Michelet. Execs at Warners felt Michelet's score wasn't powerful enough so they replaced it with an original score by David Buttolph.
When the radio announcer is reading the news about the monster's rampage through New York, various shots of the city are shown, mostly with panicked citizens in the street. When the announcer mentions the situation at Times Square, the accompanying footage shows the Palace Theater, whose marquee reads "Judy Garland - Live and in Person."
(at around 38 mins) Prof. Elson (Cecil Kellaway) reads an article about a 1797 professor who was fired for claiming that he had seen leprechauns remove a tree. He continues "Today it's monsters instead of leprechauns". Tom Nesbitt (Paul Christian) then enters and asks "How certain are you there were no leprechauns?" Cecil Kellaway played a leprechaun in the 1948 film "The Luck of the Irish".