These vicious cannibals are an unholy fusion of Chaos Dwarf and giant, rampaging bull.
Their origin can be found in the Time of Chaos when a few dwarfs found even their mighty constitutions and resistance to magic overwhelmed, leading to this horrific mutation. Now each generation a few of these 'blessed' dwarfs are born to the Dawi Zharr- usually at the expense of the mother's life.
They are taken at birth and raised by the Sorcerers who use their dark arts to further tamper with them, fusing into armour and contraptions of daemon-empowered and enfused metal. This tampering means that over time a Bull Centaur's flesh will harden and take on a consistency of living metal, leaving the Centaur requiring poultices of molten mercury, steel sutures, brazen splints etc. to heal. Naturally only their Sorceror masters can offer this.
The Bull Centaurs, though few in number, serve as temple guardians and vicious shock-troops in times of war. They require no payment for their duties as they are utterly devout to their Father in Darkness and a large number of the slave-sacrifices sent to Hashut's temples will in fact be tithed to the Bull Centaurs, who indulge their ferocious flesh-hunger in an orgy of violence.