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. He discarded the idea after making a clay model of it, and eventually settled on the reptilian Ymir.
One of the reasons the film takes place in Italy is that Ray Harryhausen always wanted to vacation there but could never afford to go on his own.
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 Ymir roars in the film are variations of elephant roars sped up and modulated in pitches at different rates.
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".
Ray Harryhausen:
a man feeding peanuts to the elephant that later battles the Ymir. He did so because the actor scheduled to play the part didn't show up. He later appears in a crowd fleeing the zoo.