The "evil centaurs" are called Centigors
"Centigors are a disturbing cross between four-legged creatures, such as horses and oxen, and the bipedal beasts of Chaos, merged together by the warping powers of the Wastes some time in ages past. They possess the hindquarters and forelegs of their quadruped ancestors, granting them great speed and strength, but the upper body of a humanoid with which they wield brutal weapons.
Strong of limb and fleet of foot, These Beast-centaurs are powerful creatures. However, they are not especially agile, and while they have great strength they lack the dexterity to manipulate objects with any skill or control. Centigors are bitter and spiteful, resenting their awkward nature, and harbour a deep jealousy of creatures whose minds and bodies are better matched. Centigors live on the northern and eastern edges of the forests of the Old World, where the trees reluctantly yield to the sparse grasslands of the Northern Wastes. They are nomadic, without settlements or even encampments of any kind, finding what little protection they need in the lee of cliffs and natural rock shelters. Often they will join the warband of a Chaos Champion, for the rewards of violence and plunder from such an allegiance are great."
From the Warhammer Official site.