A duck!

[Gregory]

An aquatic bird that, in [medieval times], was used to find out if a person was a witch or not.

This stems from the very logical idea that if a person weighs the same as a duck, then that person is [made of wood] (because both ducks and wood float in water). And since wood burns (just like [witches]) then that person must be a witch, because witches are made of wood.

Therefore, the accused person(s) would be placed on a scale next to a duck, and if they balanced, the person(s) would be burned.