A Metamorphmagus is a wizard or witch born with the ability to alter his or her appearance by will. They may change the color of their hair, their facial features, or basically anything.
Unlike Animagi, Metamorphmagi have a variety of appearances to take after, while Animagi can only take the appearance of the animal they are able to transform into. It is an ability that comes at birth and may not be learned by being taught. Metamorphmagi can possibly be inherited from ancestors, as Teddy Lupin was a Metamorphmagus, just as his mother, Nymphadora Tonks, was.
 Known Metamorphmagi
 Nymphadora Tonks
Nymphadora Tonks was born with her Metamorphmagus abilities, although it is not known where she inherited them from or if she simply received them. She often changes her facial features and the color of her hair. When she was troubled with her feelings about Remus Lupin, she had difficulty being happy and altering her appearance as she usually did. She took advantage of her ability and changed the color of her hair to a white color when she was bringing Harry to 12 Grimmauld Place to disguise herself as an old lady, which shows that they can be useful for disguises.
 Teddy Lupin
Teddy Lupin must have inherited his Metamorphmagus abilities from his mother, Nymphadora. He showed signs of his ability at an early age by constantly changing the color of his hair. He was born with blond hair, but he changed it to black, eventually to ginger, and sometimes turquoise. It is not said that he altered his facial features as his mother often did, but it is possible he might have.