Well all I know is what I've been told. Supposedly the mountain is haunted by 'The Big Grey Man of Macdui' or 'Am Fear Liath Mor.' Usually just your usual oversized ghostly figure or whatever, though more recently someone told me they'd heard of a pointy eared, talon footed, horned humanoid ghostly figure running behind a car on the road once. Weird.
There are also stories of other unusual phenomena too. People claim to have heard echoing footfalls, malevolent voices, unearthly music and sensations of unease and terror. However, sightings and suchlike have apparently decreased.
One time I overheard an exchange in a pub in the area. (What? You can't do go around the wilderness and not have a drink at the end!) Anyway, someone was asking what had happened to the 'Big Grey Man' and the bartender told them "Haven't you heard? Macdui's too overcrowded these days so he's gone to live on..." And I don't remember the name of the mountain. It was in Gaelic. I think it was a neighbouring mountain. Not that it matters anyways.
And that's all I know. It's unique as it's the only mountain with such reports. Personally I'm not one for believing in ghosts and hauntings. Past experiences have left no room for petty fears and phobias.
And besides that, one of the bothies I stayed at a few weeks ago was supposed to be haunted by the ghost of a ghillie who hung themselves there, but I never saw or heard any of the alleged signs such as footsteps at night and the front door jamming shut. If anything, it'll be someone complaining about the door being too narrow, which it was.