Glaucus

Glaucus was a mortal man who was turned into merman, and became a sea deity.

Biography
Glaucus was a regular human and fisherman by trade. He lived in the Boeotian city of Anthedon. After he had found and eaten a magical herb that brought the fish he had caught back to life, he turned into a merman. This forced him to live in the sea. At first Glaucus didn't want to accept his fate, but Oceanus and Tethys quickly accepted him among the other sea deities. From them he learned the art of prophecy.

In one version of the story, Glaucus is the reason why Scylla is turned into a sea-dwelling monster. Glaucus falls in love with beautiful human Scylla, who is repulsed by him. Glaucus asked for a love potion from Circe, but Circe fell in love with Glaucus herself. But when Glaucus rejected Circe, and still loved Scylla, Circe poisoned the pool where Scylla bathed and transformed her into a creature she was known.

Trivia

 * He is also described to have fins for hands and tail for legs.