Beorn is a secondary yet pivotal character in J.R.R. Tolkien's fantasy classic The Hobbit.
Some say that he is a bear descended from the great and ancient bears of the mountains that lived there before the giants came. Others say that he is a man descended from the first men who lived before Smaug or the other dragons came into this part of the world, and before the goblins came into the hills out of the north.
Beorn first appears in the seventh chapter of The Hobbit (titled "Queer Lodgings"), and makes an appearance in the Battle of Five Armies. He's an arctanthrope - Gandalf calls him a "skin-changer". He's human by day, bear at night. And not just a normal human or bear - in both forms he's huge, while not gigantic.
Beorn lives in a great hall between the northern Anduin and the western edge of Mirkwood. As a human, he's a natural loner and somewhat distrustful of others, although he is "good". His animals are his companions; he lives with intelligent horses, dogs, sheep and such, who can do housework and communicate with him.
As a bear, Beorn seems to have many companions. Thorin Oakenshield, Bilbo Baggins and the rest of the adventurers from the story stay at Beorn's hall as guests for three nights. Each night, Bilbo wakes up to the sounds of bears outside, and each morning Gandalf finds many beartracks, as if there were a great "bears' meeting".
As mentioned above, Beorn also shows up in the Battle of Five Armies at the climax of the book. Wargs and goblins led by Bolg son of Azog attack the armies of elves, men, and dwarves, which are led by Gandalf, Thorin Oakenshield, Bard the Dragon Slayer, Dain and the Elvenking. The forces of good are greatly outnumbered, even after the Eagles join the battle, until Beorn himself - as a bear - enters the fray and decimates the goblin rearguard. Without his help, the dwarves, elves and men would likely have lost, which would probably have changed the backstory to The Lord of the Rings. Furthermore, when Thorin is injured, Beorn saves his life (for a day or two) by pulling him out of the battle.
Beorn is mentioned briefly in The Lord of the Rings, which takes place about 75 years after the events of The Hobbit. Beorn's son Grimbeorn has taken his father's place as protector of the Wild between northern Mirkwood and the Misty Mountains, and is also the chieftain of a group of Men known as the Beornings.