I read the Gargoyle entry in the new Monster Manual yesterday.
Basically, gargoyles are elemental in origin, born out of the cracked footsteps of the prince of elemental evil Ogremoch (who made his oddly compelling but oddly illustrated appearance in the Fiend Folio.) They're cruel bullies who'll side with evil beings pretty much because it gives them a chance to be cruel bullies.
In Felltower, gargoyles are also elemental in origin, born out of the warping effects of Earth magic on the dungeons of Felltower. It's not clear how they become cruel bullies out of that, but it's possibly the effects of Felltower and not of magic in general.* They spring up twisted from fragments of magically shaped rock, and inflict their personality issues on anyone they can and will freely side with evil beings to do so.
I just feel really sad I didn't think to tie the winged elemental guys to Ogremoch in my own game.
But who knows, maybe he's the reason why shaped stone turns into winged mockeries of flight with cruel intentions.
* Although, we have it on the authority of the party wizard that all wizards are evil.