"Mystery".
This word is used for anything or situation that cannot be given a logical explanation: lights in the sky, people who have disappeared into thin air, strange sightings of non-human creatures. Perhaps at times it is too abused and risks turning something normal, or very serious, into a television phenomenon.
Many are afraid of what has no explanation… but just as many are those who, instead; they are fascinated by it.
A half-human, half-fish creature that lives in the depths of the sea and that only emerges on the surface on rare occasions, both for simple reasons of curiosity and for hunting purposes ... what could be more magical and mysterious?