Aos Sí (pronounced like Eess She): Any spirit from the Otherworld including the Tuath Dé, Fomorians, and faeries of both the Seelie and Unseelie Courts.
Cù Duibh (pronounced like Koo Duff): A mythical hound with a shaggy black or dark green coat that is usually unnaturally large with glowing red or yellow eyes. They are sometimes associated with electrical storms and also with crossroads, barrows, places of execution, and ancient pathways. They are capable of hunting silently, but occasionally let out three terrifying barks, and only three, that could be heard for miles by those listening for it, even far out at sea. Those who hear the barking of the Cù Duibh must reach safety by the third bark or be overcome with terror.
Dobharchú (pronounced like Door-who): a monster that resembles an otter but said to be about five times as large (perhaps 10–15 feet), with white pelt, black ear tips and a black cross on its back. Though, due to the murky waters it is said to reside in, its pelt may be portrayed as darker.
Fuath (pronounced like Foo-uh): The term "fuath" refers to a class of malevolent spirits inhabiting the sea, rivers, fresh water, or sea lochs, with several "subspecies" falling under it.
Goblin: The term is common in English, Scottish, and Irish folklore, the name serving as a blanket term for all sorts of evil or mischievous spirits. The word goblin is first recorded in the 14th century and is probably from the Old French gobelin, attested around 1195. Like elf, fairy, pixie, and puca, folk belief had many names and types of supernatural beings, whose definitions are often fluid, and trying to hold to strict definitions when working with folk informants often leads to frustration and confusion.
Otherworld: This is the realm of the deities and possibly also the dead. It is a supernatural realm of everlasting youth, beauty, health, abundance and joy. It is described either as a parallel world that exists alongside our own, or as a heavenly land beyond the sea or under the earth. The Otherworld is described as elusive in myths, but various heroes visit it either through chance or after being invited by one of its residents. They often reach it by entering ancient burial mounds or caves, or by going under water or across the western sea. Sometimes, they suddenly find themselves in the Otherworld with the appearance of a magic mist, supernatural beings or unusual animals. In some stories an otherworldly woman invites the hero into the Otherworld by offering a silver apple branch, or a ball of thread to follow as it unwinds.
Seelie and Unseelie Courts: The Seelie Court is a group of “good” fairies who contrast with the wicked Unseelie Court. The Seelie Court are those fairies who would seek help from humans, warn those who have accidentally offended them, and return human kindness with favors of their own. Still, a fairy belonging to this court would avenge insults and could be prone to mischief. Conversely, the Unseelie Court are the darkly-inclined fairies who may attack without provocation.
Unseelie Slóg: Literally, “unhappy army,” these are evil spirits or creatures from the Otherworld who invade this world.