This is a horn core. A cow has horns which are made of Keratin, the same material as hair and nail. Inside the horn is a bony core which supports the horn and it is made of bone, this is what you have found. The center of the horn core is hollow and in life would contain the blood vessels and nerves which feed the horn. Sheep, Goats, Giant Ox; any cattle with horns will have similar horn cores.