Mizzo (ミズオ, Mizuo) appears as a cut enemy in Adventure Mode: The Subspace Emissary in Super Smash Bros. Brawl. It can only be found in a display room in the Battleship Halberd Exterior, along with other Subspace enemies, and can be seen attached to two blue tentacles and attempts to swim around.
Its appearance and motion (alongside how the trophy description specifies its "swimming motions") suggests that it was originally going to be an enemy encountered in the water, as does the fact that its name comes from the Japanese word "mizu", which means water. In spite of being cut from the actual Subspace campaign, it does possess in-game animation files, though most are bugged and result in limbs tearing across the screen or crashing.
As Mizzo is not found outside of the display room in Battleship Halberd Exterior, it is among the few enemies in the Subspace Emissary that cannot be obtained via the Trophy Stand. Instead, the Mizzo trophy is randomly found wherever trophies can be spawned, such as the Coin Launcher and the All-Star Rest Area.