"tales of the piskies and the spriggans, of the black dogs of the moors and the seal-women of the channel."
Black dogs are commonly found in folklore, especially in British legends. One such tale inspired Arthur Conan Doyle's story The Hound of the Baskervilles.
Piskies is another name for pixies, often found in folktales from Devon and Cornwall, where Essie Tregowan is from.
Spriggans are sprites local to Cornwall. They are often described as hideous creatures that like to cause havoc.
The seal-women are Selkies, shapeshifting mythical creatures that can shed their seal skins and become human. But if someone steals their pelts then they are trapped in their human form. This is a variant of the swan maiden myths.