Chaos energies tend to radically warp most creatures who come into contact with it, usually in oddly ironic and disturbing ways. Chaos mutation can almost be said to have a sense of humor about itself. 

While mutation is normally considered random and chaos mutation would be thought to be doubly so. There are some standard mutative types which the chaos gods, Tzeentch in particular, has grown fond of. As such, Beastmen, the more thouroughly mutated forms of mutants take on a series of standard forms, albeit with a great deal of internal variation and a propensity toward strange growths, tumors and tendrils.