Beorn is unlocked to purchase after completing Chapter 9, “Queer Lodgings.” He is located around his home and costs 45,000 studs.
Fast travel to Beorn’s Homestead which along the western edge of Mirkwood. When you land, he will be waiting. Walk up to him and press to purchase him. Next, bring up the character menu and select him. To transform, hold down and he will transform into a bear, unlocking this achievement.
Or, if you don’t want to spend the studs or wait until after Chapter 9, you can enter in the cheat code ‘KEID2V’ at any time and he will automatically be purchased and select-able.
There is also a creature customizer here where you can unlock “Who is this horrid creature?”