In the year 520 AD, a gigantic
dragon splashed out of the
Seine river in France and started making quite a sodden mess out of
Rouen - unlike most dragons, known to expel blasts of flame with their reptilian breath, this one instead gushed forth great rushing torrents of
water, resulting in the
drowning of peasants,
flooding of agricultural land and generally making a large nuisance of itself, snacking on the occasional serf when it got tired of altering the ambient
humidity level. On account of its unique form of attack, the inhabitants of the region named it the
Gargler, which en francais is Gargouille.
Eventually growing tired of the new beachfront property his cathedral had become, the Archbishop of Rouen St. Romain decided to do something about this moist menace and set out to round up a posse to bitchslap this wet wurm. This endeavour was mostly unsuccessful - everyone was afraid of getting wet (remember that they didn't have baths too often at this point in history) and ultimately the only person the Archbishop could round up willing to help remove this pest was a condemned murderer who had nothing to lose (save his grimy facade).
This odd couple made their way down to a cave by the banks of the Seine where the monster was rumoured to abide. Sure enough, they found it there blowing bubbles to amuse itself. Seeing lunch (mmm... sacrilicious!) approaching the dragon reared up out of the water to chow down on its visitors when the Archbishop made the sign of the cross, the sight of which completely took the fight out of the beast.
Some time later St. Romain marched proudly into town with the murderer leading the dragon on a leash, at which point the townspeople set upon it and tossed it upon the biggest pyre ever seen, stoked with every dry piece of wood in the entire province. The Archbishop pleaded for merciful treatment of the monster, citing scientific research as a motivation for keeping it alive when the whole "mercy" thing didn't play over too well with the dripping masses, but when the townspeople couldn't see how the dragon's ability to safely transport its young in its belly could improve their lives they shrugged their shoulders, exclaimed "C'est la vie!" and proceeded to make a nice dragon flambé in a white wine sauce. (Just kidding about that sauce part.)
His feelings of kindness unable to be effected upon the horrible monster, the Archbishop instead redirected them to his murderous companion, who was fully pardoned and set free, and from that day on a tradition arose that every Ascention day the holder of the post of Archbishop of Rouen could pardon one criminal.
The notion of the gargling monster stayed in the French popular consciousness such that even today stone replicas of the dragon can be seen on the corners of gutters atop cathedrals, gargoyles vomiting forth excess rainwater.
Because I don't want to make a separate writeup for this small quantity of quasi-related information, keep in mind that monstrous sculptures which don't serve water-removal purposes aren't called gargoyles but instead grotesques.