Sure, you definitely would have found it on a normal playthrough if it were already added by then.
It's in the forest ruins where you runa way from the boulders and need the form that lets you shrink to enter it.
There is a new chest there.
Viewing post in Field Effects and one new Bad Ending added comments
Sorry, seems like I misunderstand!
Nope only indirectly since it isn't very hidden.
When Nana finds the Treasure she makes a comment about like: "That would make for a good offering."
But maybe adding a hint for it sounds like a good Idea that I may use.
Maybe I add one new buyble hint, where you get the transformationg hints.