In both D&D and Pathfinder canon, the origins of the owlbear are unknown and contested. Some say it is the result of a wizard's twisted imagination. Others claim it is a prototype create of the Fey, the First World, and has somehow migrated to the Material Plane (the main world we play in).
Out of game, the owlbear is credited to Gary Gygax, co-creator of D&D.
The owlbear as a monster, lives and hunts primarily in forests and wooded areas. They can strike alone, or in groups of two to six. Or more, if the dungeon master/game master (DM/GM) requires more.
Since I play Pathfinder more than D&D, some things may not cross over fully. That said, in their current PF iteration, owlbears are given a base challenge rating of 4. So a party consisting of 3-4 members should start to run into one owlbear at around level 3. (Average Party Level +1 should equal CR for a challenging fight.)
Owlbears are fiercely territorial and will attack anything that comes into their area. They can be tamed, by skilled (or lucky) druids or rangers, but most of the time the owlbear will attack relentlessly until defeated.
The attack with razor sharp claws, gashing at their enemies. Or by grabbing an enemy and forcefully holding it in place while it bites and stabs with its beak.
Owlbears aren't particularly intelligent animals, at least at base level, but aren't mindless either. Should one be given intelligence through the spell Awaken, they'd be an even deadlier threat.
The beastiary lists variants of the owlbear, including the higher-level and deadlier Siege Owlbear, but with how customizable games like D&D and Pathfinder are, the GM can adapt the creature to suit the needs of the story.
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