Adaro: ghost, merman, and sprite of the Pacific islands

The adaro is a malevolent ghost in the folklore of the Melanesian. When a person dies, their better half or aunga passes on whereas their worse half or adaro remains behind. The adaro becomes a malevolent ghost that assumes the form of a shark-like monster. They travel across rainbows and waterspouts, and kill men by throwing fish (often described as flying and venomous).

According to Encyclopedia of Beasts and Monsters in Myth, Legend and Folklore by Theresa Bane, they lived on the sun but visited earth by riding rainbows and waterspouts during sun showers. They could be friendly, visiting people in dreams to teach song and dance. They were often malicious, knocking people unconscious and only waking them if placated with an offering of a flying fox. Their chief was Ngorieru, who haunted the shore of San Cristobal.

Like a lot of folkloric monsters, their traits varied wildly by region. Students of comparative mythology may note superficial similarities to the hún and pò of Chinese folk religion.

In Pathfinder, they became malevolent merfolk with shark features. 3.x statistics emulating the folklore were posted at The Worlds of Mankind.

I find the original Melanesian monster way more interesting than a generic shark-themed merman, mostly because it doesn't pretend to have a pseudo-naturalistic ecology by virtue of being a ghost and its habits of crossing rainbows, riding ocean tornadoes, and throwing venomous flying fish sounds pretty cool. Finding a flying fox to cure an unconscious victim sounds like a pretty quick plot hook.

"Adaro/Anguiliian" by Azel/Atolm