Besides gargoyles, the most famous of which are of course the Notre Dame gargoyles, there are chimeras (equally grotesque but not architecturally useful as spouts for rain water). But I love mascarons, especially this one above a door in Thonon-les-Bains, France, photographed by a friend:
This little monster looks so aggravated in his attempt to scare evil spirits away, rolling his eyes and showing teeth, reluctantly, like a disgruntled dog. I love this example of a monstrous creature because he (is it really a gendered one?) seems to be a cross between a traditional mascaron, a Green Man, an animal and a sun-shaped ornament, but especially because of his intense expression and individuality.
These monsters in stone are a fascinating feature of popular culture, originating in antiquity but most ubiquitous in the Middle Ages. A clean wall or façade were a perfect slate for expression of the tastes of the community rather than a “cultivated” artistic individuality. Indeed, a gargoyle wouldn’t be included in a church’s façade if the community wouldn’t recognize it as a meaningful element. I wonder what thoughts and emotions those mascarons evoked in people entering or passing by a building. They probably weren’t intimidated but just fascinated with their grotesqueness. Feel free to suggest links of interesting monsters!