Beorn was a skin-changer, a man who could assume the appearance of a bear. He lived with his tame horses in a wooden house between the Misty Mountains and Mirkwood, to the east of the Great River of Wilderland. His origins lay in the distant past, and Gandalf suspected he and his people had originally come from the mountains.
Beorn named the Carrock and created the steps that led from its base to the flat top.
During the Quest fo Erebor, Beorn received Gandalf, Bilbo Baggins, and the thirteen Dwarves and gave the Dwarves and Bilbo help in their quest. In the Battle of Five Armies, Beorn rescued Thorin Oakenshield from the Goblins and killed their leader Bolg.
By the time of the War of the Ring, Beorn had become a leader of Men which included other skin-changers, and woodmen. His people were known as the Beornings, and they helped defend Thranduil's kingdom of northern Mirkwood. He died some time before the War of the Ring itself began, and was succeeded by his son Grimbeorn the Old.
Beorn is the Old English word for a bear, and sometimes was indeed used as a name. It is related to the Norse name Bjorn which, in turn, may suggest a connection to berserkers.