Yeats divided faeries into the solitary and trooping faeries, as did James Macdougall in Folk Tales and Faery Lore. Katharine Mary Briggs noted that a third distinction might be needed for “domesticated faeries” who live in human households, but such fairies might join with other faeries for merry-making and fairs.
The trooping faeries contain the aristocracy of the fairy world, including the Irish Daoine Sídhe. They are known as trooping faeries in Celtic lands because they travel in long processions, such as the one from which Tam Lin was rescued. But the trooping faeries also include other faeries of lesser importance; a trooping faery can be large or small, friendly or sinister. Unlike the trooping faeries, solitary faeries live alone and are inclined to be wicked and malicious creatures, except for beings such as the brownie who is said to help with household chores.
In non-Celtic lands, trooping faeries are any faeries who live in groups. They are associated with faery rings and with green hillsides, faery paths, woodlands and meadows. Sometimes they are regarded as controlling a particular woodland or expanse of water, or as caring for the environment in that area. They are smaller than the noble faeries and almost inevitably have a trickster element, though they also reward favoured mortals at whim. Many of these trooping faeries are regarded as elf-like people who practice crafts, and are mischievous rather than malevolent, challenging the status quo and causing sufficient chaos to prevent stagnation.
In Gotland, the largest of the Swedish islands in the Baltic Sea, with a population of about sixty thousand, small beings called Di små undar jord live under the earth. They wear grey or blue clothes with small caps, probably like the pixie hat—originally hoods worn by the local, people—and are said to be the size of small children. These beings leave flattened circles in fields, like mini-crop circles, in the centre of which mortals still leave coins, milk, linseeds, beer or salt to prevent or cure illness.
Original picture found here.