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The beast was a large humanoid, fish-like creature. It was green with a yellow belly, fin, and had glowing eyes.
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Powers and abilities
It was an exceptional swimmer and could breathe underwater.
The Scooby-Doo Show
Mystery Inc. arrived on a fishing trip to find the village deserted, except for the owner of an convenience store, a mysterious fisherman, and Julie. They were told that the lake level was falling, strange blasts filled the night, and mysterious lights could be seen in the Lake.
It wasn't long until the gang encountered the beast. After several chases (including one in which Shaggy Rogers and Scooby-Doo were blown up with helium) Fred Jones decided to scuba dive under Bottomless Lake. Shaggy and Scooby stayed behind, as usual, and promptly encountered the Beast.
It chased them into the lake and an underwater base. There they found that someone had blasted a tunnel in the lake floor. After getting sucked through an airlock, they were caught in a net by two henchmen. Fred and the others arrived in time to catch the others and unmask the Beast who was Julie. She disguised herself as the Beast to cover up the fact that she was shipping stolen gold through the tunnel to her henchmen. Julie and her helpers were then taken to prison.