I believe there is no such a monster in the lake @matajingga. To my opinion, there are peoples who doesn't like visotors come to that place. They are the spiteful peoples, envy and jealousy. People from the community around the lake may take advantage by selling food and baverages such as warung nasi or kedai kopi around the lake. The spiteful people hate that. Hence, they made the rumors, then as you see now, visitors abandoned the lake