I sometimes see these sinister-looking pods at Chinese markets and bought a few yesterday at the Berkeley Bowl. They’re a water caltrop (Trapa bicornis), native to South East Asia, and from the same family as water chestnuts. Depending on your perspective, these pods look like either the head of a bull, a bat, or the head of Satan.
Water caltrops are aquatic plants that produce flowers on tall, slim stems. The flowers then produce the bat nut, which is the seed pod of the plant.
These are toxic when raw and must be cooked before eaten. Cooking them is easy; simply boil in some salted water for 7 to 10 minutes, or roast over an open fire like chestnuts. Remove the dark outer shell to reveal the starchy white flesh inside. Eat them straight out of the pod, or add them to a stir-fry.