Someone posted something like this up a while back. I found this one a few years ago. I tend to agree with them in that something like this was likely leather wrapped and attached to a handle. Looks like they would make a bad a** whacker.
We call those potato rocks. They are for the most part natural. Some of them will show pecking or some alteration but most are natural. They drive me nuts because I can think of 5,000 uses for all of them. I have one that is football shaped and has signs of pecking on it.