For the baby bear (mama's favorite), who grows up when you hit the path to dragonmaw pass, the quest is called Tutorial Relics... the bear is a great distraction for a ranged fighter such as my dragon mage and bfg seraphim... it does crap for damage so even though you get no xp for his kills, it only manages to kill 1 out of every 5000 or so monsters. There are some creatures which it does not attack, however, and so is useless at drawing them to him for those.
As for Urzi the bear... he does way more damage than mama's favorite... and can probably kill 1 out of every 500 or so monsters I kill (this bear is bigger than mama's favorite... Although no xp for kills, this really doesn't matter at all.
I always use both of these bears for my ranged fighters at every difficulty level by not completing their respective quests. Mama's favorite takes damage but heals to full everytime it dies (not comsuming any of your health potions either), whereas Urzi never takes damage. Essentially, both are immortal.
However, some creatures are ignored by these bears, like some undead/demons. Each bear attacks and ignores a different set of monsters, does anyone feel like compiling a list of ignored? That would take some time, but hasn't been done yet as far as I know. Also, the bears do not level at anytime... they stay the same throughout the game. Urzi is actually quite strong in act 1, but basically useless later on at damaging anything.
Regardless, they are great at drawing enemy damage away from you and aggroing mobs around them (instead of you) to be blown away.