Chaos Spawn are creatures warped by the influence of raw Chaos. Even corpses, when exposed to infinite possibility, will warp and reanimate as mindless half-living mutants. Often people, when exposed to Chaos, will take on the aspects of animals and beasts, sometimes according to their personalities.
The Chaos Engineers experience a simmilar warping phenomenon due to their travel through the Second Ether in Scaling Ships. However, they are essentially Chaos Lords and aren't mindless in the way the Chaos Spawn are.