since they said they took them from the east, and since they seem taller than people, I would say gigantopithecus which inhabited south east asia and southern china. Though that would be a long journey, early in the manga the author took creative liberty about species present, adding some species that would've been extinct but having the characters say "well the fossil record is incomplete, maybe they survived for longer". Though Gigantopithecus were closer to orangutans and these don't look like the reconstructed gigantopithecus.