"I bet it sneaked up here from Experimental Charms, they're so careless, remember that poisonous duck?"
-- young female pamphlet-maker (DH13)
Poisonous ducks are ducks which can poison someone.
The Committee on Experimental Charms had at least one poisonous duck in its offices in the Ministry of Magic headquarters. Due to carelessness, a poisonous duck made its way from Experimental Charms to Level One (DH13).
Technically, an animal is poisonous if you ingest it and it causes you harm, while a venomous animal is one that bites, stings, or otherwise actively injects a harmful substance into an enemy (source: Smithsonian.com). It may be that Rowling mixed these words up just like the rest of us and meant that these ducks produce a harmful chemical when they bite or it could be that these ducks are harmless unless eaten. -BB