That was what was great about Harryhausen. He gave each and every one of his creations distinct and individual personalities-just like his mentor Willis O'Brien (Kong) did. I have no idea how they were able to accomplish that with small bendable models-but it was a tribute to their genius. It's something this CGI crap can't ever do. God, I miss stop-motion photography(curse you, Lucas & Spielberg)! And, yeah, I always saw the similarities between the Ymir & Kong myself. Maybe he did it as a homage to his mentor-who knows? Wonder where they came up with that name(Ymir)? It never had a name in the movie-just "the creature." By the way, he sure grew fast for hardly eating anything. :D
Maybe Forry Ackerman came up with it (Ymir).
Incidentally, Harryhausen is a staunch conservative Republican. His producer, Charles Schneer, was an equally staunch liberal Democrat, and actually helped to break the Hollywood blacklist. I guess you can think of Harryhausen and Schneer as the Stewart and Fonda of special effects.