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The Merrow is an enemy in Final Fantasy Dimensions. It is encountered in the Challenge Dungeon of the The Dark Knight part. Since the Challenge Dungeons do not appear outside the original phone release, it is only dummied in the other mobile versions and therefore not available in any English version of the game.
The wiki does not know what this enemy looks like or its family. Merrow appears in Final Fantasy V as a Sahagin, however the name "Merrow" for that enemy is a creation of localization and that is not its name in Japanese. The enemy abilities indicate away from Sahagin.
A true appearance of Merrow has appeared in another game, Final Fantasy XI, where it is a Lamia. This is a possibility, although Final Fantasy XI usually isn't the source for enemy names in this game. An alternative possibility is that this enemy is a Mermaid enemy which matches closer to the etymology of Merrow. The abilities could support either Lamia or Mermaid. The two known mermaids hold spear items and are weak to Lightning though, and this enemy matches neither of them. Its items do match those of the Lamia quite perfectly however.
Merrows are the Scottish and Irish Gaelic equivalent of the mermaid and mermen.