The standard explanation for the owlbear in-universe is "A Wizard Did It." Which is fine, as far as it goes, because the trope of "madman tinkering with bizarre mutations" is part of our speculative fiction ethos.
But what fractured mind actually came up with putting together the owl and the bear? In real life, I mean, as something to put in a D&D book? Mechanically it's just a funny-looking bear--it's big, fuzzy, has claw and bite attacks, and lives in the woods. What inspired the owlbear? If it was made up out of whole cloth, whose brain did it spring from?
And why does it look like that? A simple image search suggests that's possibly the least frightening possible depiction of the terrifying blend of bear and owl.