Huh. I’d consider that bad DM etiquette to not just say “they’re monstrosities now. Would you like to do something else instead?”
I think I'd call for an appropriate skill check to get that knowledge. DC 10 Arcana check, or Nature check.
That is an interesting distinction, and sort of suggests a familiarity with the "fantastic" elements of the setting.
Who are the PCs? Are they farm kids from a remote village suddenly out in a big bad world full of new experiences? Or are they people integrated into the fantastic cultures that exist in the world? How much do casters know about their own magic? Stuff like that.