I wrote a short blog here about these manger balls / chogs a while ago.
They are variously known as chogs, sliders, manger balls, manger logs and tying up bobbins. They were traditionally used to weight a rope that passed through a ring or slot in a manger and attached to the halter or headcollar, thus allowing the horse to move but preventing the rope from looping dangerously.
It has a good aged patina with several scars and marks from use. There is an old chip inside its central channel at one end
