A kind of early form of the merman, the Abgal is mentioned in Sumerian mythology.  It is one of a number of spirits, originally servants of Ea,  the god of wisdom.  Like the Centaurs of Greek mythology who helped civilize humanity, the task of these beings was to teach the arts and sciences to humanity.  They did this during the day while fasting, only stopping to eat at Night.  Early carved reliefs show them men Above the Waist, Fish below.