The story
From this animal's large size and ghostly appearance.
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: A species of deep-sea jellyfish, Stygiomedusa gigantea, commonly found in the ocean's midnight zone, and twilight zone usually at depths of 6,665 m (21,867 ft). It is rarely seen, but believed to be widespread throughout the world, with the exception of the Arctic Ocean. Full definition →
From this animal's large size and ghostly appearance.
Etymology from Wiktionary, CC BY-SA 4.0.