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The Stam Bear
Bears have one big advantage over other tanks: naturally high HP. Almost any bear will have higher health than any pally, warrior or DK in the raid. The stam bear takes advantage of this by pushing the envelope even further, stacking up all the stamina they can get to amass whopping amounts of health. Obviously, taking this route gives tanking bears amazingly high HP, therefore helping them last longer when engaged in boss fights. At the same time, however, they're going to be take a heck of a lot more hits.
The Dodge Bear
Feeling a little bit rogue, or just want to screw with your leather-wearing DPSers? Some bears choose to stack agility, dodge and defense in order to pad their one and only mitigation stat. High dodge helps bears deflect and defend against blows, which helps negate the need for massively high HP. Stacking nothing but dodge, however, means those blows will hurt a whole more when they do land...and eventually, they will land.
The Hybrid
The hybrid bear focuses on both stam and dodge (really, the only two things a PvE bear can focus on) to achieve fairly high percentages in both health and mitigation. This is the route most bears take as a matter of course, achieving a combination that allows them to both absorb and deflect blows as they come. Gearing hybrid helps bears remain versatile in the game, as well, as this can be somewhat beneficial in PvP as well.