A Metamorphmagus is a witch or wizard in the Harry Potter continuum who can change their appearance at will. This ability is very rare, with the series containing only two Metamorphmagi: Nymphadora Tonks and her son, Teddy Lupin.
The Metamorphmagus ability has some level of genetic heritability, shown by Teddy. However, neither of Tonks' parents were Metamorphmagi.
"Metamorphmagus" is a proper noun in Harry Potter, and the first "M" should always be capitalized.