Board Thread:Theories and Speculation/@comment-1932060-20171201013626/@comment-33156027-20171204120822

Hashirama propably had Sage Mode from Shikkotsu Forest or some other unknown place because his Sage Mode didn't looks like toad or snake senjutsu. We don't know what species of animals live in Shikkotsu Forest besides one slug, so it could be anything. Secondly, myouboku Mount, ryuuchi Cave and Shikkotsu Forest are most famous places where ninja can learn senjutsu, at least according to Kabuto's words, so there may be more less known places where shinobi can become a sage of different animals (hawks, dogs, maybe even turtles/tortoises).