Nymphadora Tonks and her son, Teddy Lupin, are currently the only known Metamorphmagi in the series; it is a very rare ability. Tonks is known to change her hair colour and style in accordance with her mood. Indeed, she even appears as an old lady on occasion. Her son, Teddy Lupin, also inherited this trait, as his hair is mentioned repeatedly changing colour.
The extent of these appearance-altering abilities and the limits thereof are not entirely clear. According to Rowling, a Metamorphmagus can alter his or her appearance completely, for instance, from black to white, young to old, handsome to plain and so on. However, it is unlikely Metamorpmagi can change their appearance into anything other than human, as this is never mentioned in the books, nor do we see Tonks change her appearance in anything that is not human, completely nor partially (although this is seen briefly in the first film she appears in, but the films are a separate canon). Though she does change her nose into "a beak-like protuberance" and "a nose like pig-snout", both are still human - the first resembling Snape and the second reminding Harry of a female Dudley.In Half-Blood Prince, Nymphadora Tonks temporarily lost her power as a Metamorphmagus after suffering sadness over her love for Remus Lupin, who wanted to distance himself from her due to him being a werewolf.It proves that a witch or wizard's emotional state can affect their inherent abilities.