The great Black Bears of Mirkwood were friends to Beorn, a skinchanger who had the ability to transform into a large black bear, who would often talk to the creatures of Mirkwood. 

Beorn's origins are unknown. He may have been akin to the first Men who lived in Northern Middle-earth. Gandalf speculated that Beorn may once have lived in the Misty Mountains. Beorn was familiar with the tunnels under the mountains and he hated the Orcs who lived there. He vowed that one day the Orcs of the Misty Mountains would perish and he would return. 

Source: Tuckborough.net