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

Dimensions : Height 3" (7.8cm), Diameter 3¼" (8.5cm)
Weight 465g (1lb ½oz)

Ref : Bobbin010