I think that is a runty, misshapen potato. Perhaps one of your girls found it growing underground and decided to take it to the nestbox.
I could be wrong, but unless it has a shell, I'm thinking potato.
Just an oddity that sometimes happens. Perhaps one of your hens was becoming eggbound or something like that. Unless it happens again or one of your hens seems ill, it probably isn't anything to worry about and you likely won't see anything like it again for a long time.