"I bet it sneaked up here from Experimental Charms, they’re so careless, remember that poisonous duck?"
-- a young witch in the Ministry of Magic (DH13)
A duck is type of waterfowl found in both fresh and sea water environments, whose vocal cries are called “quacks”.
The reaction of Herbert Chorley, a muggle Junior Minister, to being the subject of an Imperius Curse was to believe that he was a duck and make quacking noises. The Muggle Prime Minister had thought it might be a drinking or overwork problem. (HBP1)