Despite the common modern perception of fairies as lovely and helpful folklore offers a wide range of dangerous Fairy beings who represented a real threat to any humans they happened to encounter. I'd like to offer a list of 7 such dangerous beings here, although there are of course many more than that found across the folklore. These are definitely not the sorts of fairies one would want to find in one's garden.
- Redcap - a malicious type of goblin the redcap gets his name from the hat he wears which is dyed red with human blood. He is known to live in ruins and will attack humans unfortunate enough to cross his path. Unlike many other fairies he isn't averse to iron - in fact its said his shoes are made of iron or iron toed - but he will flee at the sight of a Christian cross or at Christian prayers.
- Each Uisce - literally 'water horse' these are shapeshifters who can take the form of a human when they choose to but are most often in the form of a horse. In both Irish and Scottish folklore the Each Uisce will appear on land and lure a human into riding them, only to run back to the water, drown, and eat them.
- Nuckelavee - found in Orcadian and Scottish folklore these monstrous beings live in bodies of salt water. They look like a horse with the torso of a rider on their back, the head rolling bonelessly, the arms hanging down unnaturally long; instead of hooves the horse has flippers and the entire creature is skinless. The Nuckelavee avoids fresh water, including rain, but will roam the strand near the sea and kill any living thing it finds there. It has also been blamed in folklore for droughts and for illness among horses.
- Hags - a type of water fairy found across English folklore, usually under specific names like 'Peg Powler'. Hags lurk in waterways and drown the unwary, particularly children; by some accounts they eat the people they drown. Usually described as emaciated elderly women with talons or iron tipped claws.
- Slua Sidhe - not a specific individual fairy but a grouping of them, Slua Sidhe means 'fairy host' or fairy army and is a collection of malicious Otherworldly beings who travel through the air. They may snatch up a human they run across or else may cause illness, death, or madness to those they encounter.
- Mare - The source of the modern word nightmare the mare or mår is sometimes also called a hag (not to be confused with the water ones) and attacks people while they sleep by perching on the human's chest causing sleep paralysis and night terrors. Occasionally those she torments do not survive her nightly attention and she is known to kill both humans and animals.
- Baobhan Sith - Her name means, roughly, 'evil fairy woman'. In Scottish folklore the Baobhan Sith lurk in forests and appear to hunters who express loneliness, offering to keep them company. Several accounts discuss a group of hunters out at night in the woods who meet a group of Baobhan Sithe - seemingly just human women - and invite them to join them for some music or dancing. One hunter eventually notices something is amiss and realizes the women have killed his companions and flees into the night taking refuge among the horses whose iron clad hooves ward off the fairy women. The Baobhan Sith kills by draining her victims blood and in some accounts by ripping out his heart.
John Henry Fuseli, 'The Nightmare"