The Chapter of the Elf island opens in volume 35 with Guts' group getting stranded on a lone, desolate island after an encounter with a ghost ship. It demonstrates the dangers of the new world our protagonists find themselves in. During the course of this little adventure, they meet Isma, a member of a race of beings known as Merrows.
In Japanese, the word used is 人魚 with メロウ as furigana. 人魚, which is normally pronounced にんぎょ (NINGYO), simply means mermaid or merman. メロウ (MEROU) gives us the word's intended pronunciation in Berserk: Merrow. The name is borrowed from Irish folklore, where merrows appear as a race of sea-dwelling beings.
Its most famous story seems to be "The Lady of Gollerus", where a green-haired merrow marries a man but eventually returns to the sea after having given him three children. A story not unlike that of Isma's mother, although the similarities are only superficial, as is usually the case with Miura's inspirations.