These chickens have been items since Smash 4. They’re from the Legend of Zelda games. When Link accidentally attacks one too many times, they call all of their chicken friends and they will try to murder Link.
I think it’s a hilarious idea for an item and one I’d knew what it’d do before I ever saw it work.
You just throw the chicken at your opponent and let the magic work.
While not nearly as painful as it is in the Zelda games, it can cause you to easily lose a life/match if you do get hit. Shields and evading are pretty much your only hope against them.