Monster Origins discovered ( no doubt )

[FlawlessHappiness]

I'm sorry to say it, but i thought it was kinda obvious that they were transformed by Alexander.
I recall the servants shouting "Alexander, let us out". "He left us down here with all the wine", in combination with "My chest is going to burst" made it clear that the wine was the reason for the mutation. I'm pretty sure that is what is written in the letter, down in the wine cellar.

FG made it quite clear, also because having monsters in the story with no clear origin doesn't really make sense.