A Minotaur is a Servant Beast found in the Rockshard Rampart. According to a text found in that Tower, the Minotaur was created based on a Certan myth about a beast which was half man half bull. An armoured form appears in one of the final rooms in the Truegold Tower.
During the day, this creature will stand on four legs and charge at you, altough this is quite easily avoided. It is strong enoughto break through walls. However, if you return to this Tower at night, it will stand up when you enter the area so that it is on two legs. It will then swipe at you with its arms. If it hits you it will knock you back quite a way.
Day or night, the best way to defeat the Minotaur is to use the Oraclos Chain to grab its head/body and then tie it to another one or the same one before attacking it with your weapon. As soon as the chain is thrown off just grapple it again. Or, if you prefer, you could pull away from it and then yank when the Chain becomes gold.