Wereducks

Legends persist in Überwald of the existence of a malevolent sub-set of normally human-shaped characters who can, at will, transform into the shape of ducks. Opinion is varied as to the efficacy or the usefulness of this ability. apparently those blessed with this ability become considerably more stupid and agressive, and travellers in remote areas have experienced the dread of passing thorugh a damp area and hearing the ominous quacxk and the slap of webbed feet behind them. Advice is for them to hurry away and on no account look behind them. It is also alleged that there are wereduck vampires, but the shape-of-the-beak problem makes them lamentably bad at it.

Of course, the authority for this is The Celebrated Discworld Almanak.