Turnskins are Beastmen who were born fully Human, but at some point in their lives, they warped into a new form. There’s a slight distinction between the common Chaos Mutant and the Turnskin, largely because the latter develops a total physical change rather than gaining some isolated or strange mutation.
The life of a Turnskin is misery. Caught between the world of Men and the world of Chaos, and welcome in neither, they cannot remain with their former kind and must flee into the wilderness to find others like themselves. But the Beastmen see Turnskins as things less than Brays, regarding them as weak and worthless, only allowing them to run with their Warherds because they add, however little, to their strength.