Bears live in forests, often near settlements. They are known for breaking into cabins and stealing food. They have a special fondness for honey. There is also a trained bear with a trainer at the Pearl brothel in Denerim.
A Shapeshifter in the party, notably Morrigan, is able to change into a Bear once you have unlocked such an ability. Her change makes her very powerful but disables the use of using items such as Lesser Health Poultices (and greater) and Injury Kits. To regain the ability, you must transform Morrigan back to her former self by clicking default bear skill 1.
It is also possible, if your character purchases the 'Ranger' Ability from a merchant to unlock Ranger Specializations, that they can summon a bear to aid in battle. This begins as a Black Bear, but once your character has achieved the 'Master Ranger' ability, Summon Bear will summon the much larger and more powerful Brown Bear. Note: Ranger is a Rogue Specializations