A Metamorphmagus (pl. Metamorphmagi) is a witch or wizard with the ability to change his or her physical appearance at will, rather than requiring Polyjuice Potion or a spell like the rest of the wizarding population. They are extremely rare.
Metamorphmagi vs. Animagi
- "Metamorphmagi are born, not made."
- —Tonks explaining to Harry the differences between a Metamorphmagus and an Animagus[src]
Unlike Animagi, Metamorphmagi can take a wide variety of forms, changing gender, race, and age, potentially looking like anyone at all, or even just changing a part of their appearances, such as the shape of their noses. Animagi also transform at will, but only into one animal form which is determined by their inner traits. It is never revealed if Metamorphmagi can take the full form of an animal, however it is highly unlikely as its compared to a potion and specifically describes outward appearance changes.
Also, a Metamorphmagus is born, not made; it is not a skill that can be learned. There also seems to be a degree of heritability, as both Nymphadora Tonks and her son, Teddy Lupin, were Metamorphmagi, although Tonk's parents were not. The ability presents very early in life, as Teddy Lupin's hair changed colour when he was a newborn infant, as did his mother's.
Using Metamorphic Abilities
It is unknown whether Metamorphmagi receive any training in developing their innate skill, or if they simply learn through trial and error themselves; however, given their rarity, it is somewhat unlikely that much training is available.
It is unknown if Metamorphmagi must register with the Ministry of Magic like Animagi must, as Metamorphmagi did not choose to become what they are, while Animagi did. However, just like the Ministry is determined to prevent Animagi from abusing their transformation abilities, it is presumed that Metamorphmagi must also be registered for the same reasons, though this would seem unfair and easier to get by, as the said Metamorphmagi may simply deny of their abilities from the Ministry to escape registry. Though it seems illogical to assume they'd need to be registered as they would either be so similar in features to their original (or 'true') form that it would not be necessary to register them, or they would be so different - and thereby have no 'identifying marks' such as with animagi - that any attempt to register them would be futile as there'd be no way of knowing it was them.
Like many other types of magic, a Metamorphmagus' abilities are affected by his or her emotional state. The person's appearance at any moment will often reflect the emotion the person is feeling at the time. Strong emotions, such as shock or depression, can affect their abilities to the point of having trouble using them at all. For example, Nymphadora Tonks had difficulty using her abilities when she was depressed over her feelings for Remus Lupin  and following Sirius Black's death.
Behind the scenes
- Metamorphmagi can be born to non-Metamorphagi, as Nymphadora Tonks was a Metamorphmagus, when neither her mother, Andromeda Black, nor her father, Ted Tonks, was.
In Greek, "meta" means "change," "morph" means "shape," and "magus" means "magic or wizard." Hence, a wizard that can change shape, and the term "Metamorphosis" is the term to describe an animal changing into another animal such as how a caterpillar changes into a butterfly.
- Harry Potter and the Order of the Phoenix
- Harry Potter and the Order of the Phoenix (film)
- Harry Potter and the Half-Blood Prince (Mentioned only)
- Harry Potter and the Deathly Hallows (Mentioned only)
- Pottermore (Mentioned only)