I do NOT use creamed horseradish on oysters. I prefer prepared horseradish. Horseradish is the root of the horseradish plant.
You can prepare your own horseradish from fresh horseradish roots. This may cause you to cry due to the strong aromatics. I wear a scuba mask or swimming goggles when preparing fresh horseradish. You can blend this hot radish with tomato puree for a hot cocktail sauce.