Abominations appear like an anthropomorphic snake, Malisons appear like either a human body with a snake head or a human upper body with a snake lower body, and Purebloods appear almost exactly like humans do, just with snake-like eyes and patches of scale-like skin. Yuan-Ti are a half-snake half-human race, however their appearance differs depending on their subrace. From Masters of the Forbidden City by Codo Vidak

'The Yuan-Ti cast off their humanity long ago, and with it, their sanity.' Yuan-Ti Abomination, as it appears in 5e's Monster Manual.