The Lake Monster was an ancient monster who appeared in episode 040. She was never given a proper name, but she is usually referred to as the Lake Monster. Although she was not a true youma, she is often considered to be one due to Endymion's attempt to use her in a way similar to the Dark Kingdom youma. She came from the lake near the Steamy Hot Springs Inn. Kunzite told Queen Beryl that a strange and powerful energy was coming from the lake. The queen told Endymion to investigate, which he did right away. When he arrived at the lake, he overheard Ikuko telling Usagi about the myth of the Legendary Lovers. She said, "It is an old story that's been passed down through the generations. A maiden of the Heavens had come down to Earth, and a youth of the land fell in love with her. But a girl, who adored that youth, turned into a monster in a fit of jealousy and she attacked the villagers indiscriminately. The maiden from the Heavens and the youth sealed the monster in this lake with the power of their love. And having exhausted their energy, they rose up to the Heavens together." After hearing the story, Endymion figured out that the legend was true, and that the source of the strange energy in the lake was actually the monster.

Endymion called forth the spirit in the lake in an attempt to harness its power for use by the Dark Kingdom. When it awakened, however, it was completely out of control. It flew wildly threw the air and kept repeating, "Give him back!" After seeing Usagi, the monster went after the Tsukino family. Although he was extremely frightened, Kenji attempted to protect his family by standing up to the monster. The monster was able to easily knock away the stick he had picked up to defend himself, and she began choking him. Luckily, Endymion arrived dressed as Tuxedo Kamen and threw a black rose through the air which hit the monster and loosened her grip on Kenji. The Sailor Senshi (with the exception of Sailor Moon) arrived just in time to rescue the family. Sailor Mercury used Sabão Spray to obstruct the monster's vision and confuse her. While the Tsukino family made their escape through the fog, Usagi stayed behind and transformed into Sailor Moon.

Sailor Moon clapped her hands to get the monster's attention through the mist. After giving her speech, she told Tuxedo Kamen that he was a good person for protecting Usagi. Unfortunately, this caused her to lose her concentration, allowing the monster to attack her with its tail. To stun the monster, Sailor Venus quickly used her Crescent Beam attack, while Sailor Jupiter used Supreme Thunder. Although the monster initially disappeared, it quickly reappears. Sailor Moon uses Moon Tiara Action attack, causing it to disappear and reappear once again. The Senshi could not believe that their attacks were not working to kill the monster. Sailor Mars soon correctly realized that this was no ordinary monster: she was an evil spirit "born of old fanatical jealousy." Endymion confirmed what she said and told them that he was the one responsible for awakening the monster. He told them to back off, but Sailor Mars said that this was in her domain. She used her Akuryou Taisan attack to place an anti-evil scroll on the monster, then used Fire Soul to completely stun it. Sailor Moon took the opportunity to use Moon Healing Escalation, which healed the monster and caused her transform back into the young girl she once was. She had finally been freed of her ancient jealousy, and she ascended into the Heavens. Before she went, she waved back to Endymion, who smiled and waved back.

(Source: The Oracle :: BSSM Encyclopaedia)