It is not known to evolve into or from any other Pokémon.
Stunfisk is a flat, fish-like Pokémon with a broad, mud-brown body. At the base of its yellow tail is a yellow marking shaped like an exclamation mark when seen from behind. It has small, horizontal, oval-shaped eyes, white with a tiny black pupil and no irises. Almost exactly between these two eyes lies the two thick gray lips that form its mouth. It has two small, flimsy fins at the sides of its body; each with two lobes. It has been seen to leap into the air and flap its fins as if they're wings to remain airborne.
Stunfisk's very tough skin keeps them unharmed if stepped on, even by sumo wrestlers. To exploit this, it buries itself in mud of the seashore and waits for prey to pass by. Whenever unsuspecting prey touch a Stunfisk's body, it is zapped. Stunfisk smiles when electrocuting its foes. If it cannot paralyze its foe, it contorts itself with unexpected speed and flops away.
Stunfisk seems to be based on a flounder or dab combined with an electric ray. It may also be based on the stargazer, another fish that spends most of its time buried in the sea bottom and can make electric discharges when disturbed.
Stunfisk may be a combination of stun and fisk (fish in most Northern European languages, from Old English fisk) or a combination of stun, fish, and disk (in reference to its flat shape).
Maggyo may be a combination of マッド maddo (mud) and 魚 gyo (fish). It may also refer to まぐろ maguro (tuna).