It has been implied in Manga and multiple theories that Hashirama's SM originates from Shikkotsu Mountain like Jiraiya + Minato + Naruto's originates from Myoboku Mountain and Kabuto's originates from Ryuchido Cave. Any different opinions/theories.
That's an extremely nice point. Most learners learn to summon the animal they learned Senjutsu from. There's a hidden twist to this it seems.Katsuyu doesn't have wood style so it's hard to say that it's where Hashirama's Sage Mode comes from, especially when Hashirama can't summon her either.
Great post, his teacher's not probably an animal but a Buddha sage.Probably the one with a thousand arms sticking out of it.