There is much to be said about glaistigs, and much more about gruagachs, and many snippets of stories about both, but I shall try to confine myself to some bare essentials so that we are not here all day.
Both glaistigs and gruagachs are found across Scotland, and there is some overlap where the names may be used interchangeably or their natures may overlap, but in other cases they are quite different.
Glaistigs
Glaistigs appear as women, sometimes human sized, sometimes smaller, often dressed in green and with golden hair. On occasion they may take an animal form, particularly that of a goat, or appear as half goat, half woman. In some accounts, glaistigs were human women adopted by the Folk and given fairy nature and powers.
Glaistigs were sometimes associated with water, but more often with a particular house or neighbourhood. Usually, glaistigs were found around farms and houses, undertaking domestic duties or herding cattle. They took those in need under their protection, particularly old people or children, and were often accompanied by a small child as they went about their work.
Glaistigs who looked after the cattle expected a libation of milk to be provided for them, and this was often left in a hollow in a rock and many different traditions surround it in different areas - such as providing a small amount of milk every night, or all the milk from one cow at the start of each season, not looking back once the milk has been left, leaving milk any time you must pass the rock while carrying milk, and so on. As with any of the Folk, a glaistig might become unpredictable if she felt she had been disrespected, and might punish those seen as disrespectful, or leave the district altogether.
While a glaistig could be murderous and dangerous, this is more when they have been provoked than as an essential part of their nature. There is some suggestion that glaistigs might take the form of a beautiful woman to lure a man to his death, usually one known to him, but this suggestion is leveled at many women of the Folk and the story given (of glaistigs seducing men and leaving them with their throats cut and drained of blood) sounds suspiciously like on more often attributed to the baobhan sith.
It is important not to forget to leave out the libation of milk for the glaistig. And if you have to pass her something, you may want to pass it on the tip of a dagger or something else made of cold iron so that she does not grab your hand and take you away with her - although I warn against trying to trick her into hurting herself, as stories tell she will curse you and all your line.
A glaistig herding cattle.
Gruagach
In the highlands, gruagachs may appear as fairy ladies dressed in green with long hair, often acting as the guardian of cattle. They were associated with water, and were known to come into houses dripping wet and ask to warm and dry themselves by the fire. These are clearly closely related to the glaistigs, and some accounts suggest they are one and the same.
Unlike glaistigs however, gruagachs could also be men - in some areas of the highlands they were handsome, slender young men who wore red and green, and in others they were mostly short, shaggy, and largely naked. Both kinds of gruagach would help out on farms, and like the glaistig would expect a libation of milk to be left out for them, although the young men in particular might also amuse themselves by letting the cattle out of their pens at night and laughing at the farmer.
Finally, the name gruagach was also given to some creatures of immense power, who were more of a giant, a wizard, and a king - and stories are told of humans both challenging and aiding them.
Sometimes a gruagach would come dripping to the fire to dry herself.
Story time
The following are two short, sad stories of both a glaistig and a gruagach that died in helping a human family, but each one indicates the strong relationship between humans and these Folk and how much they were appreciated.
The Grogach of Ballycastle
In Ballycastle there was a farm at which worked a grogach, a spirit much like a gruagach or a brownie - described as having a low stature and a hairy body, with broad shoulders and being very strong.
There was a custom in the farm for the farmer to lay out a number of sheaves of corn in the granary overnight for the grogach to thresh by morning, but one night the farmer forgot to lay out the sheaves and instead left the flail on top of the whole stack of corn.
Alas, the grogach took this to mean that he was to thresh the entire stack, and in the morning he was found dead on top of the grain. The farmer gave him an honest burial and he was long mourned.
The Glaistig of Mull
On the island of Mull there lived a glaistig who was very attached to the Lamont family, and looked after their cattle. Sometimes she would take the form of a dog to better look after the cows.
One day, raiders landed on the island intending to take the cattle and the glaistig took on her dog form and drove the cattle up into the hills away from them. The raiders kept coming, however, and in desperation the glaistig turned all the cattle into great stones so that the raiders could not carry them off.
Once the raiders were gone, however, she could not get the cows to turn back into their original form, and in remorse she drooped and died, despite the best efforts of the family to cheer her and return her to health. The family buried her with respect beside the Sound of Mull.