Ray got his inspiration from watching the original King Kong in 1933, which featured the animated ape himself along with several prehistoric creatures. The stop motion animation scenes were done by Willis O'Brien. Harryhausen eventually met O'Brien who became his mentor and friend, launching him into the film business by working several films together in the 1950's. He then carried the torch for a few decades more.
My first look at his work was trailers for the Sinbad movies in the 70's and I was hooked. I later saw Jason and the Argonauts on TV (thanks to my Dad) and it became for me the one film I always associated with Ray's work. Talos, Harpies, the Hydra, etc. It didn't matter that the stop motion animation was obvious - the look of the characters themselves (which Ray created himself) combined with the way they interacted with the live action was quite enough to leave an impression on my memory. The skeleton army battle is, I believe, still unparalleled. Add to that the rich story lines of Greek mythology and you have the making of a classic.
Below is a list of some films that Ray Harryhausen contributed to that have at least some connection to science fiction. I'm only including the ones that I've seen so far so I can say a few words on each film, but I will also try to include video of some of the stop motion scenes. I think most of these would be considered "B" material today.
20 Million Miles to Earth (1957) - There's not much to this story. A kid finds a space egg (from a crashed rocket returning from Venus) that hatches a monster which rapidly grows into a giant and terrorizes the town. Actually, it's more a monster film than science fiction. (With blogger's new video policy I could only get this sample clip)
One Million Years B.C. (1966) - This one is famous first for showing hours of Rachel Welch in a toga. It's supposed to depict life in cave man times, but it is quite comical because the cave people act like modern actors who want to say their lines but just can't talk. But I had to include it because it has some of the best Harryhausen dinosaur fights on film. The one below is a classic example. The shots from the perspective of the humans watching from below probably inspired Steven Spielberg during the making of Jurassic Park.
The Valley of Gwangi (1969) - I discovered this one late and I think it has some merit. In some ways it parallels the King Kong story. There is a valley in Mexico known to be forbidden by the locals although they don't know why. An expedition group finds it is a lost dinosaur haven and go in to bring back an Allosaur. They succeed but, as the locals had warned, it comes back to bite them literally as the dinosaur escapes into the city. A neat aspect of the film is it represents a crossing of two eras. It runs like an old Mexican Western with cow ranchers, horses, and rodeos, while at the same time being a traditional monster film. A great illustration of this is the landmark scene of the ranchers roping the Allosaurus like it was a wild bull! Ray later said the matching of the real ropes to the animated ones was difficult to achieve. The film also has a neat scene of the dino roaming it's way through an old Cathedral. (With blogger's new video policy I can only show the trailer below).