Monsters of Gallic myth and legend

[cmacq]

What about stuff like the banshee or other things monstrous, rather than simply otherwordly?

Not to draw too fine a point,
but the banshee was not Gallic, and using a strict definition of the Classical term, they were not Celtic. However, many Gaulish and Gallic mythic elements were later morphed into somewhat monstrous fabrications within the framework of Judeochristian monotheism. For example the Gaulish/Gallic Ogmios, or 'the silver tongued (devil),' elements of which were incorperated into the modern Christian concept (in part) of Satan (a character that could be viewed as a monster of sorts).