Ancient Creature of the Lake is a sea monster. Instead of legs, she has a long dragonlike tail. She appeared in Sailor Moon Episode 36, Last Resort.
She was originally a girl who got jealous and was transformed into a sea monster known as Ancient Creature of the Lake. She was awakened by Prince Darien, but he was unable to control her. She attacked the Sailor Scouts, but nothing they did seemed to work, not even Sailor Moon's Moon Tiara Magic. In the end, she was healed by Sailor Moon using Moon Healing Activation, turning her back into the spirit of the girl.
Ancient Creature of the Lake is not a Negamonster, rather a sea monster awakened by Darien.
The way she became a monster, as well as Serena's line "I hope I never get so jealous that I turn into some creeped out monster", is similar to the way Emerald, jealous of Sailor Moon over Prince Diamond, receives a dark crown from Wiseman to become his queen, and is instead turned into a dragon.