Xbox 360 / PS3 / PC: BiPolar Bear can be obtained at the end of Snow World. To obtain him, the Horn is needed. Blow the Horn in front of the cave leading to Ice Castle, next to the horn sign, and BiPolar Bear will emerge, allowing the player to obtain him.
BiPolar Bear will maul any player, character, or enemy of any sort with less than 11 HP. The fact that he mauls even his owner seems like a bad thing at first, but over time players develop skill and work around it, because most of the time the chances of your health bar being at or below 11 HP are astronomical; in simpler terms, BiPolar Bear is only bad for you if you've an embarrassing lack of skill.
A nice bonus to this is that he'll prevent overkill because you'll know exactly when an enemy is dead. Many experienced players tend to overkill with their juggles because they'd rather play it safe than sorry, and BiPolar Bear trims that process down to speed to make playing efficient all around, and he even is as efficient for farming as he is normal gameplay because of this.
BiPolar Bear will maul any boss with less than 11 PERCENT of their HP left; notice that's a percentage, not total. BiPolar Bear is the single best pet to bring to any boss fight because of this. You get XP every time he hits the boss, so even though you're not attacking the boss you still reap the benefits. BiPolar Bear does 32-96 damage per hit; hitting 3-4 times a second.
"Weapon Effect Usage"
Like Hawkster, BiPolar Bear can freeze, shock, burn, poison, and critical hit.