I think it's some sort of radish thing. I peeled it and took a bite, and while I was temporarily overcome with horror at that "you really don't want to eat this" radish smell, I actually ended up liking it. Like most vegetables, I would have totally dreaded this thing as a kid, but now I'm able to get over the unpleasant stinky first impression and appreciate it as a very different flavor from the norm. I suspect this would be interesting grated and mixed into a salad or used in a sandwich as one might use a pickle or onion. It was surprisingly spicey, in a way similar to raw garlic, but more intense.
Update: It was a Black Spanish Radish.