It's something of a mystery for me, and many others here in the West who have discovered Horus, Prince of the Sun, just who animated that spectacular opening scene. Yasuo Otsuka is best known for the scene with the giant fish, but not necessarily this one. Hayao Miyazaki, naturally, poured himself into every nook and cranny of the movie, but, aside from the two action scenes that were shot in a rapid-fire still photo montage, I don't know which scenes were his.
Thankfully, the Otsuka documentary, Joy in Motion, solves this riddle for of. Well, sort of. The film opens at a gallery retrospective of Otsuka's career, which features many animation cells from the many productions he worked on. As the camera pans along the walls, you can spot several cells from Horus: that giant fish again, and three poses of Horus, charging the camera in the opening scene.
However, later on, when discussing the movie, the narrator cites Miyazaki as a key animator for the scene. This makes a certain kind of sense; Otsuka and Miyazaki were the two "action" animators of the period, so I've long assumed one of them was responsible. The documentary shows Takahata, Miyazaki, and Otsuka posing as Horus swinging his axe (even here, you can see how intuitive Miyazaki and Otsuka are with swift movement). So it makes sense that they were both responsible for the opening scene.
I think my only reservation about Miyazaki's involvement is that nothing in the opening two minutes of Horus ends up riffed in his later work. That's something you'd almost expect to see by now. He has quoted this film, notably two shots in Howl's Moving Castle, and a shot of Hilda that became one of his iconic images in Nausicaa. Shows what I know, but that's why I keep searching for answers. It's always fun to discover something new that you hadn't seen before.