The Skin-changers

Skin-changers, also known as Shape-changers, were Men and rare individuals with the ability to assume the form and abilities of an animal.

History
In the Third Age, Beorn (who dwelt in the north-eastern Anduin Vales) was a Skin-changer who helped Bilbo Baggins, Thorin Oakenshield and his Company during their Quest to Erebor. He took the shape of a huge black bear and a black-haired man, yet the origin of this ability is unknown. According to Gandalf, Beorn "[was] under no enchantment but his own."

It is possible that Beorn's son Grimbeorn was a Skin-changer.