The bear is an ultimate ability for the Warden class. You need to create a Warden character, and then level that skill line to level 12 and you will be able to unlock that ultimate ability.
To be clear, you need to get the animal companion skill line to 12. Actually reaching level 12 as a character won't unlock your ultimate. The bear is also kinda underwhelming. It's a neat concept, but not necessary.
If you picked up the collector's edition or preordered, you should have the option to change your bear's skin under collectibles. That will switch it between normal the brown/orange bear and a grey variant. Unlike with werewolves, the color change does not denote an actual difference in the ability; it's strictly cosmetic.