You are never unarmed. Your hooves are a natural weapon, which you can use to make unarmed strikes. When you make an unarmed strike with your hooves, the damage die for your unarmed strike is a 1d6+4.
If the centaur moves at least 30 feet straight toward a target and then hits it with a weapon attack on the same turn, the target takes an extra 1d6 piercing damage.
The centaur makes two attacks: one with a melee weapon or spell and one with its hooves or two with a ranged weapon or spell.