In the middle of an unimportant combat with some bandits a burst of wild magic transforms all of the PCs and their opponents in to random animals and monsters. They retain their intelligence (though not, of course, the ability of speech). They can either carry on the combat in their new forms, panic, or otherwise react how they see best. After about an hour, they return, unharmed, to their normal form.
The greatest tree in the woods has a secret, there is a dungeon under it made from the intricate web of roots. What creatures could reside here? What secrets will the PCs uncover?