Batnut, Devil Pod, or Root of Gargoyle?
Ian was first to respond with an excellent citation to Cook's Thesaurus. Water Caltrop is the English name for this type of pod. For the etymology of "caltrop" (creepy!) see Yucan's note.
Horned water chestnut, ling kok, ling jiao, and various other names are used for this black nut that does look like a bat or like a bull's horned head. Interestingly, they are toxic if uncooked.
So, Ian gets the glory for first answer and correct answer. Also reminding me of a good food information resource: The Cook's Thesaurus.
Jesse, Michael, Sharon, Jim, Larry, Graham, Anne, Yucan...good job! Nice to know I've got a small army of food geeks on my team!
And Marisa, I really do like the Root of Gargoyle...you get the glory for most imaginative name!