Legendary, huh... And what would its associated legend be?
Missingno, filling the role of hellion adversary to the god Arceus, becaue every proper deity needs a nemesis. While Arceus is a god of order and creation, legend has it that Arceus's arch-enemy Missingno is a force of pure chaos and destruction, sometimes appearing as the skeletal remains of long-dead Pokémon, sometimes appearing as white noise incarnate: its five forms are known as Kabutops Forme, Aerodactyl Forme, Ghost Forme, Tetris Forme, and Matrix Forme.
While myths abound that those who stumble upon it go insane and lose their mind, or that it is the ruling deity over a realm known as Glitch City (a location found in Kanto myth, similar to Sinnoh's myths of the Torn World; perhaps they are associated), the only truly malevolent action Missingno is known to have taken is cause malfunctions in the PCs of trainers who have become Kanto Champions. And thus, modern historians doing research on the myths surrounding Missingno are forced to conclude that the legendary Missingno is not a truly "evil" Pokémon but, more likely, a trickster Pokémon that gets an undeservedly bad reputation due to its fear-inducing appearances, not unlike the reputation most Ghost-types earn.
In addition, more recent evidence has suggested that Missingno is, at times, benevolent. When young boys and girls find themselves facing a Trainer or a wild Pokémon without any Pokémon of their own, a strange Unown-like Pokémon in the shape of a floating symbol (suggested by many to be a mysterious sixth form of Missingno, Question Forme) will apparate out of the air to defend the child--and while it is easily defeated by any Pokémon it faces, its appearance and subsequent self-sacrifice allows the child to flee to the refuge of a nearby Pokémon Center. However, also supporting the "trickster" theory, when children who are saved by the mysterious Pokémon (possibly Question Forme Missingno) enter a Pokémon Center, it will have terrible malfunctions with the automatic Pokémon healing machine, causing it to throw Pokéballs wildly around the room.
... I had way too much fun with that.