Tony DiTerlizzi covers the origins of the Chinasaurs very well here. I don't think I need to rehash his words, so I will get straight into the game talk!
The reason why I have chosen the owlbear this week is because I recently placed one in Mord Mar. And it is a problem. The party refused to go near it! They trudged off in another direction. And, really I don't blame them. Here's the Bounty Board Wanted Poster. The bad photoshop was intentional.
The owlbear is a tank. With three attacks, 5+ Hit Dice and a hug attack, these creatures are quite dangerous to low level parties. Most grognards remember the owlbear in B2 quite well. Usually that owlbear killed 1-2 party members on my expeditions.
This owlbear is a prime example of why older editions didn't use challenge ratings. This monster would kill everything it could. But, it had a weakness that could be exploited. And that weakness doesn't play well into a CR. That weakness: it is dumb. Did the party run into a pit trap on the way to the owlbear? Have it chase you back to the pit trap, and stab it from above. Did you just slaughter a few goblins? Give the corpses to the owlbear, and he may let you go on your way.
In the courtyard of the citadel, an owlbear has made a nest. As this is the main transition from one level to another, she has a steady supply of food. Even more appetizing for her are the goblins that keep coming out of the citadel to see if she is gone.
A dryad in the Bitterbark Forest has charmed an owlbear. She often has it attack the woodsmen from Redstone. The merchant's guild offers a 500 gp reward for dealing with the creature. But, if the adventurers kill the owlbear, the dryad will never forgive them.
Although unintelligent, an owlbear has figured out that rust monsters make things easier to eat. She has made a lair across the hallway in an abandoned castle from a rust monster clutch. They often ambush the 2-legged creatures together that enter their combined territory.
The goblins of the Palneal Swamp captured a live owlbear. Their leader had delusions of riding the beast, but it ate him. Now, the goblins believe the owlbear to be their leader, per tradition. It walks among them, occasionally eating a goblin. The clan fell into disarray without leadership, and sent their bravest warriors to the Oracle of Stone and Flame to ask for guidance. They returned with only the advice "bring the humans in to kill it." So a group of goblins has shown up at Redstone's gates pleading for "strong human warriors." They offer the tribe's greatest possession "The Stick of Boom" (a wand of lightning bolts, with 6 charges left.)
The good picture of the owlbear is by Dave Allsop, bought on RPGNow. Ironhead Arts is the publisher.