"If you get caught in the Boss Galaga item's tractor beam, it will carry you away to the top of the screen," Sakurai wrote. "You can shoot it down, but it won't join you in battle for double the firepower."
For those too young to recall the early arcades, the Boss Galaga made its debut in the 1981 hit Galaga. Unlike other Galaga critters which sought only to destroy the player's ship, the Boss Galaga attempted to draw the ship in via tractor beam. If the Boss Galaga stole a player's ship and was then killed, the ship it swiped would be combined with the player's existing ship, doubling its effective firepower.
As Sakurai mentioned, none of this takes place in Super Smash Bros., but at least now you know why the pixelated bug is attempting to abduct Donkey Kong.