Frank Frazetta’s cover for Carson of Venus by Edgar Rice Burroughs.
The waters often teemed with fish; and occasionally I saw monstrous creatures of the deep, some of which defy description and would challenge belief. The most numerous of these larger creatures must attain a length of fully a thousand feet. It has a wide mouth and huge, protruding eyes between which a smaller eye is perched upon a cylindrical shaft some fifteen feet above its head. The shaft is erectile; and when the creature is at rest upon the surface or when it is swimming normally beneath, it reclines along its back; but when alarmed or searching for food, the shaft springs erect. It also functions as a periscope as the beast swims a few feet beneath the surface. The Amtorians call it a rotik, meaning three-eye. When I first saw one I thought it an enormous ocean liner as it lay on the surface of the ocean in the distance.
Another (slightly plagiarized) version: