Merascylla is possibly taken from Greek Mythology's Scylla, and according to Greek Mythology, Scylla is a sea monster who lived underneath a dangerous rock at one side of the Strait of Messia, opposite the whirlpool Charybdis.
Scylla was a nymph, daughter of Phorcys. The fisherman-turned-sea-god Glaucus fell madly in love with her, but she fled from him onto the land where he could not follow. Despair filled his heart. He went to the sorceress Circe to ask for a love potion to melt Scylla's heart. As he told his tale of love to Circe, she herself fell in love with him. She wooed him with her sweetest words and looks, but the sea-god would have none of her. Circe was furiously angry, but with Scylla and not with Glaucus. She prepared a vial of very powerful poison and poured it in the pool where Scylla bathed. As soon as the nymph entered the water she was transformed into a frightful monster with twelve feet and six heads, each with three rows of teeth. Below the waist her body was made up of hideous monsters, like dogs, who barked unceasingly. She stood there in utter misery, unable to move, loathing and destroying everything that came into her reach, a peril to all sailors who passed near her. Whenever a ship passed, each of her heads would seize one of the crew.
Now it states that she is a nymph, daughter of a God, so technically she is from Goddess Clan. This also states how Merascylla was transformed into a monster from Circe's sorcery, possibly Merascylla was also tranformed into a monster by someone's sorcery, possibly Demon King or someone else? That remains a mystery for now but I'm quite sure we'll get to see who corrupted her soul.
Also if I may add, Merascylla's Commandment was "Faith". Faith is something associated with Gods not demons.
She is also known to resurrect the dead, Gods have that ability, demons don't, and once earlier Goddess Clan offered Ban to kill Meliodas in exchange for Elaine's revival. Although Merascylla's revivals were corrupted, maybe because her soul is corrupted in the first place.
Like Dolor(Giant Clan) and Gloxinia(Fairy Clan), we didn't belong to Demon initially and infact fought them 3000 years ago in the war of five clans but later joined them for reason or another, maybe they were corrupted by someone, the same person who corrupted Merascylla, since I don't see them joining the Demon Clan by their own accord.
So what do you guys think, do post.