Glaucus was a Sea-God. He is known for patronizing fisherman and sailors in storms since he used to be a fisherman himself.
In the Greek mythology there are several stories which tell the tale of him becoming of a God. The first one claims he was born as a fisherman and he found a magical herb that brought a fish he caught back to life. Curios, Glaucus ate the magical herb. The herb made him immortal, but turned his arms into fins and his legs into a fish's tail, making him a merman. At first he disliked being a God but the titans Oceanus and Tethys found him and he was quickly accepted by the other Sea-Gods who taught him the art of prophecy. The other one is that he was born a God-Child of Nereus.
In another myth, Glaucus loved a beautiful Nymph named Scylla. But when he approached her, she was appalled by him because of his fish tail so she fled to land where he could not reach her. Glaucus went to Circe and asked her to make Scylla love him. But instead Circe fell in love with him and told him to forget about Scylla and start loving her. But Glaucus said to her that trees would have to grow underwater and seaweed would grow on land before he stopped loving Scylla. Circe, angry and bitter by rejection, poisoned the pool that Scylla bathed in so that she would turn into a monster with twelve feet and six heads.
Glaucus is described as a blue-skinned merman with copper-green hair with a scaly fish tail where his legs should be.