An octopus is neither a fish nor an amphibian; it actually belongs to a class of animals known as cephalopods, which are part of the mollusk phylum. Unlike fish, octopuses have soft bodies and do not possess a backbone. Instead, they have a complex nervous system and highly developed brains.
Fish are typically characterized by their gills, fins, and a streamlined body, which octopuses do not have. Amphibians, on the other hand, usually go through a metamorphosis from a water-dependent larval stage to an air-breathing adult form, like frogs and salamanders. Octopuses are fully aquatic and do not undergo such a transformation.
In summary, the unique anatomy and life cycle of octopuses distinguish them from both fish and amphibians, placing them in their own category within the animal kingdom.