It was voiced by the late Pat Stevens.
The Beast of Bottomless Lake was a creature that Mystery Inc. encountered when they 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. It was a green fishlike creature with arms, legs, glowing eyes, and a yellow belly and fin. After several chases (including one in which Shaggy and Scooby are blown up with helium) Fred decides to scuba dive into Bottemless Lake. Shaggy and Scooby stay behind, as usual, and promptly encounter the Beast.
It chases them into the lake and an underwater base. Here 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. It was Julie, who 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.