The monster of Odail Pass on the Isle of Skye, and one of the Highland demon spirits. It is described by J.G.Campbell in Witchcraft and Second Sight in the Scottish Highlands (p.207-208). It seems to have been a nasty creature to meet.

Sometimes it bore the form of a man, sometimes of a man with only one leg; at other times it appeared like a greyhound or beast prowling about; and sometimes it was heard uttering frightful shrieks and outcries which made the workmen leave their bothies in horror. It was only during the night it was seen or heard.

It was not only horrible to see and hear, it seemed to be hunting for blood to appease it.

It ceased when a man was found dead at the roadside, pierced with two wounds, one on his side and one on his leg, with a hand pressed on each wound. It was considered impossible that these wounds could have been inflicted by human agency.

The distinction between demon spirits and demonic ghosts is hard to draw, and people might well have accounted for Biasd Bheulach as the ravening ghost of a murdered man, hungry for revenge.