MOTD Phantom of the Lake

While at a hot springs resort, Usagi's mother tells her the story of how a girl that became jealous of two lovers turned into a monster and started attacking people until the two lovers were able to trap it at the bottom of the lake. Endymion overhears this story and later he uses his power to revive the monster. There is a huge splash in the middle of the lake and a serpent-like monster rises up out of the water and heads straight towards Endymion.
Sailor Moon Episode #40
The lake of Monster Legend! Usagi's family tie
Mizuumi no densetsu youkai! Usagi kazoku no kizuna