Osschaart (also: Ossaart, Osschaard) is a waterspirit or shapeshifting demon from the Flemish and Dutch folklore.
Osschaart can take the shape of any animal he wants, but he usually appears in the shape of a bull with a man's head and chains in his hands.
He jumps on the backs of passengers, mostly drunkards, at night. In other stories he jumps on boats or backs of fishermen, especially when they forgot to toss back the first catch into the water. His presence could be detected by the sound of rattling chains.