|Associates:||The Legendary Lovers, Evil Prince Endymion, the Inner Senshi|
|First Anime Appearance:||The Legendary Lake Monster! Usagi's Family Ties|
|First Manga Appearance:||N/A|
|First PGSM Appearance:||N/A|
The Lake Monster was not a Youma, but rather a vengeful monster whom Evil Prince Endymion tried to control. She was once a normal woman who was rejected by the man she loved for another woman, a couple who became known as the legendary lovers. Her rejection and hatred transformed her into a monster and she attacked the village they lived in, but was sealed away in the lake by the legendary lovers.
Prince Endymion had thought to use this monster, but he only succeeded in unsealing her, whereupon she immediately went on a rampage and attacked Usagi Tsukino's family. Usagi was nearly forced to transform in front of her family to protect them, but the Inner Senshi appeared and managed to distract the monster, allowing Usagi's family to escape and giving Usagi the chance to transform into Sailor Moon. While the monster was distracted by one of Sailor Mars' ofuda, Sailor Moon used Moon Healing Escalation to turn her back into the spirit of the woman she once was, and she then ascended to the heavens.
The Lake Monster's lower body was a spike-edged tail that she could use as a whip, and she could also attack by breathing out a corrosive purple fog. She was capable of flying and hovering in mid-air, and could regenerate her body after being attacked.