Skin-changer

Skin-changers were Men (possibly only Northmen), rare individuals with the ability to assume the form and abilities of an animal. The only skin-changers specifically mentioned in Tolkien's legendarium were Beorn, who dwelt in the northeastern Anduin Vales late in the Third Age, his son Grimbeorn, and their descendants. Beorn and the men of his bloodline for many generations were able to take the forms of huge black bears. The origin of this ability is unknown; however, Gandalf the Grey said of Beorn that "he [was] under no enchantment but his own."

It is possible that skin-changers of other bloodlines might have been able to turn into other animals instead of bears. However, there are no recorded examples of such persons.