Since he planned to use a real elephant for some of the footage in the zoo, Ray Harryhausen asked for one that was 15 feet tall, but the film was only able to procure an eight-foot-tall one for him. In order to make the elephant look much bigger, a 4'6" actor was cast to play the zookeeper.
The film was originally going to be set in Chicago, with the rocket crashing in Lake Michigan. Right before submitting the idea to producer Charles H. Schneer, Ray Harryhausen decided to change the setting to Italy at the last minute, after deciding that he always wanted to go on vacation there.
Ray Harryhausen's original design for the monster was a giant cyclops, similar to the one he later used in The 7th Voyage of Sinbad (1958). He discarded the idea after making a clay model of it, and eventually settled on the reptilian Ymir.
Though the creature is referred to as the Ymir in reviews and websites, the name is never mentioned in the movie. Ray Harryhausen was concerned that audiences would mistake it for the Arabic title "Emir".
Upon completion of this film, Ray Harryhausen felt that, after doing destructive modern-day sci-fi monster thrillers, his interests have shifted into doing fantasy adventures (with monsters, of course) set in a romantic past, a trait that began with his next film, The 7th Voyage of Sinbad (1958). Thus, "20 Million Miles to Earth" was his last film to have a purely modern-day setting. (Only one other subsequent film of his, First Men in the Moon (1964), has bookends set in the present.) This was also his last film in black & white.
In the original press kit, Ray Harryhausen's stop-motion process was called "Electrolitic Dynamation." The name of this sales gimmick would ultimately be shortened into simply "Dynamation." (See The 7th Voyage of Sinbad (1958)).