Help needed remembering a Rupert episode

Long ago I have seen one episode of animation where a fox/bear mistakenly enters into a world of water animals, where various frogs sing and dance. In the end, while the frogs come to know there is some outsider, they vanish under the water. Please let me know if it is a Rupert episode or any other series. I have been searching this for almost 20 years.

Thanks in advance.
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