Its most likely a sage mode based on nature, its not impossible for someone to gain transformations without animals, jugo clan have transformed into forms that resemble juubi fissions and even rikudo sennin without animals, I think it most likely goes into the japanese belief of enlightenment, Hashirama with his mokuton must have become so close and one with nature that he was enlightened by the forces of nature, He became one with them..
thus we see him in that mode, summoning creatures that are keynote, divine beings in japanese culture, the Buddha statue, the dragon with the human, all of them are japanese culture