The Boss Galaga is an item, originally appearing in Smash 4. It's based on the enemies of the same name from the Galaga series. Anyone who played the classic arcade game, should remember this one, I think.
I love how its 2D look fits so oddly with the 3D graphics.
But anyway, it's a throwing item. But it mainly just hovers around and tries to take a player off-screen to their doom.
While it's a bit easy to avoid, you don't want to be caught in its homing beacon. I've rarely died against them, but they've saved me a few times.