OK, I'll run with this since I am a black belt in karate too. You wouldn't weaponize your rear, no - though the Twerking Monks of Myth Drannor brings a very amusing image to mind. Realistically though, we DO train to use any number of body parts that can be weaponized - hands, feet, elbows, knees. You learn to use your entire body for balance, power,torque and so on. It stands to reason that any intelligent race would develop it's martial arts to make use of all their available natural weapons, and for a minotaur race, that very much includes their horns. Any bar brawler learns to use his weapons - punching and kicking, head butts, etc. A monk/martial artist trains dilligently to use them more effectively.