Way back when, I asked Mrs. Spenser a question from the Sergio Leone and the Infield Fly Rule Film Quiz, "Your favourite Harryhausen film/monster?" She answered Ymir from 20 Million Miles to Earth.
We got the Blu-ray disc and watched the colorized version, but didn't listen to Tim Burton interviewing Ray Harryhausen on the extras. I think this answers another question from the quiz: "Can you think of a black & white movie that might actually improve if it was in color?" It has a nice hand-tinted postcard look, but more realistic. Not garish, just a touch pastel.
The story: A spaceship crashes in the sea near Sicily. Some fishermen get one living and one dead American astronauts out, and a little Italian boy finds a strange biological sample and sells it to a biologist and his beautiful daughter. The sample quickly grows to monstrous size and threatens (for a change) Rome.
The monster (never identified as Ymir in the movie) is a green reptile with a Harryhausenly animated tail. As in so many of his films, the monster steals all the scenes. He is also rather lovable, and possibly a vegetarian (or perhaps a sulfur-tarian). Nonetheless, when he gets big enough to fight an elephant, he heads to the Roman zoo, and ... You can guess how it ends.
As it turns out, Mrs. Spenser didn't remember this monster at all. She was thinking of something else from her deep childhood. No matter. It's a good sci-fi B movie made great by some Roman locations and Harryhausen's monster.