ladies who smoke turn into ribbons and samurais turn into genji gauntlets. (You'll have to do it multiple times because often, it seems to be more often than not, they turn into worthless items instead.

Those are the only 2 that I remember thinking were very important.

Wait, I also remember spending a tremendous amount of time using it on Cactrots. But I don't know why.

I know that you can steal a tiger mask from some common dog monster and get a really cool helmet armor type early in the game.