The Great Mighty Poo is a character in the famous N64 game, Conker's Bad Fur Day. He is a big opera-singing, Sweet Corn-eating pile of sludgy fecal matter who appears as the boss of the Sloprano chapter. He's very irritable and seems to enjoy singing and throwing blobs of fecal matter at Conker.
The Great Mighty Poo appears in All Star Smashers as a Primary Spirit, as a Master class, where you must fight Yuuki Ogata at Epic Norm Stage in order to obtain him. When fighting Yuuki, he will be giant sized, and he'll favour using his side special move, which trolls you with unusually insane effectiveness.