Metamorphmagus

"Not much hair. It looked black when he was born, but I swear it's turned ginger in the hour since. Probably blond by the time I get back. Andromeda says Tonks's hair started changing colour the day that she was born."

- Remus Lupin on his infant son's Metamorphic abilities

A Metamorphmagus (pl. Metamorphmagi) is a witch or wizard with the ability to change his or her physical appearance at will, rather than requiring a Potion or a spell like the rest of the wizarding population. They are extremely rare.

Metamorphmagi vs. Animagi
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, into any animal form, as many different animals as they wish, which is determined by their inner traits, these animal can be of any kind.

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. 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 likely that Metamorphmagi must register with the Ministry of Magic, which keeps track of Animagi.

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.

Metamorphic abilities can be very helpful when it comes to disguises. For example, Nymphadora Tonks was able to put her abilities to use in performing espionage for the Order of the Phoenix.

Etymology
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 a animal changing into another animal such as how a caterpillar changes into a butterfly.

Known Metamorphmagi

 * Nymphadora Tonks
 * Teddy Lupin

Notes and references
Nymphadora Tonks