The water caltrop (a.k.a. bull’s head, bull nut, buffalo nut, and bat nut) is an aquatic plant, native to Asia, that produces a starchy, hard fruit, with a distinctive shape, like a bull with horns. rworange says each side has a different little face.
Pei* loves them, and describes them as sort of a combination of chestnut and peanut. They have to be cooked–raw, they’re toxic. Boil them for about half an hour or more, with a few pieces of star anise. It’s actually quite hard to overcook these. They’re done when soft all the way through. Break them open, like roasted chestnuts, and consume.