Normally, it is almost unheard of for bears to attack travelers. They are, in fact, so shy and so inactive during the day that most people never encounter a bear at all. However, should a bear be provoked, they are remarkably dangerous. The normally placid-seeming creatures become enraged, and can strike massive blows with their paws, capable of knocking a man off his feet. —From Codex entry: Bear
Black bears are the most common type of bears. They can be summoned by a Ranger and Shapeshifters can transform into one. The Ranger's bear levels up with the character and thus makes for a great tanking ally, with high health and damage output. While same applies to the bear form, its use prevents the casting of spells and thus it should only be used as a last resort.
The one in the picture above. It is far larger and more powerful than the Black bear, and gains Overwhelm. Master Ranger converts the summoned bear to Great Bear, marginally increasing its power and letting it use Rage.