Ancient Creature of the Lake

Ancient Creature of the Lake was an evil creature appearing in Sailor Moon Episode 36, Last Resort.

She took the form of a serpent-like evil spirit. Originally she a woman who got jealous and was transformed into a monster which Darien refers to as the Ancient Creature of the Lake.

She was awakened by evil Prince Darien, but as she was extremely strong, he was unable to control her. She first attacked Serena, mistaking her for the water sprite that stole the man she loved. The Sailor Scouts finally arrive to help, but none of their powers seem to work; not even Sailor Moon's tiara.

In the end, Sailor Mars discovered the only way they could defeat her was to use Sailor Moon's wand. Mars weakened her with a Parchment Scroll and her Mars Fire Ignite attack, allowing Sailor Moon to use Moon Healing Activation to remove the jealousy in her heart, and allowing her be free.

She floated into the sky returning to her human form, and rising into the sky to become another happy star, just like in the story of the Guardians of the Lake.

Trivia

 * Ancient Creature of the Lake is not a Negamonster, rather an evil spirit that Darien freed from her lake seal.
 * In the Japanese version, she was simply referred to as "Lake Monster" (湖の妖怪).