Well, now that I think about it I suppose the description could be interpreted as a thestral, although that isn't what I had in mind.

The creature is actually the Jersey Devil that supposedly resides within the Pine Barrens of NJ, USA. It is kind of like the Loch Ness, with all kinds of supposed-evidence and stories behind the legend. I didn't think anyone would guess it since it is a pretty random thing, but for some reason when trying to think of an appropriate threat to Camelot the Jersey Devil came to mind.

Thanks for your comment!