mercreature means A creature of the sea, especially one which is mythical. Lexicurio rates it Sui generis — a strength score of 88 out of 100.
Etymology
From mer- + creature.
noun
- A creature of the sea, especially one which is mythical.“But there was one among them who looked different, a mercreature with arms that glowed like pearl and a purple tail, flying higher than the others, almost as if she would lake wing.”