It's not a buckeye. What else can you tell us about it. Was the outside soft with a hard center, or was it all soft, or hard? I assume you found it on e the ground. If it was in a tree or shrub pics of the plant would help. It's not something I am familiar with..... Just but the looks I would guess the center is hard like a pit of some sort - so I wonder if it's some sort of peach or plum that hasn't done well......
I am coming up with a much longer list of what it isn't vs what is is. Maybe Native Hunter knows.....