These blocks 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.

This example is turned from a single piece of golden brown very heavy wood or lignum vitae

It has a good aged patina with a few scars and marks from use and a couple of dark spots.

Dimensions : Height 3⅛" (8.2cm), Diameter 3¾" (9.5cm)
Weight 474g (1lb ¾oz)

Ref : Bobbin013