The Stimpy Fish appeared in "Prehistoric Stimpy".
The Stimpy Fish was a prehistoric fish that resembled Stimpy. Two Stimpy Fish evolved arms and legs to walk on land.
The first walked straight into a tar pit. About 150 billion years later, the other walked around the tar pit, but fell prey to the prehistoric bus Maximus Transportius. According to Wilbur Cobb, the Stimpy Fish was an evolutionary dead end and went extinct.