What is a skin-changer?

Answer 1

They're only mentioned once that I recall in LOTR, by Gandalf while he recounts a history -- they aren't in the narrative that I recall, although I think Silmarilion may touch on them. Typically, skin changers or shape-shifters are those who can at will adopt the apprearance of other persons, and in more extreme cases, animals. Depending on which legend you're talking about, they may adopt the abilities and characteristics of whatever they change into -- or they may not. JRRT doesn't go into this. JRRT's descriptions in Silmarillion of the Eldar days also included vampires, werewolves, and quite a few more typical monsters. The most famous skin-changer is Beorn, who turns from man to bear when he needed to. He is a key character in The Hobbit, providing food and shelter to the party for a few days, as well as being one heroes of The Battle of the Five Armies.

Answer 2

A skin changer is Beorn's native species. they lived in the Anduin valley between the mirkwood forest and the the misty mountains. They guarded a place called the Ford of the carrock, mainly from goblins and wargs. All their kind could change from human state into a certain animal (although you only see Beorn turn into a grizzly). over the years their very peaceful species died off. (also, they can speak to any animal in their human form)