Classification: Mythological
Origin: Melanesia
The description of these creatures was as a man with gills behind his ears, tail fins for feet, a horn like a shark's dorsal fin, and a swordfish like spear growing out of his head. (I wonder what the female version would look like.) The difference between this type of merman and others is that this particular creature traveled in waterspouts or by rainbows. (It was more likely they were seen around rainbows.) Although it is suggested that they originate after a person dies since they come from the evil part of human's soul. (With this theory it is possible for the Adaro to travel by rainbows.) A weapon these creatures use is a type of bow to fire poisonous flying fish at people.
Additional reading: Solomon Islands
Sources: http://cryptidz.wikia.com/wiki/Adaro, http://www.godchecker.com/pantheon/oceanic-mythology.php?deity=ADARO, https://en.wikipedia.org/wiki/Adaro_(mythology)