Vivillon

Due to the lack of Sleep Clause and viable Sleep Talk users, Vivillon uniquely stands out as a premier sleep inducer and Quiver Dance sweeper, able to take advantage of guaranteed sleep turns to reliably set up Substitutes and Quiver Dances due to Compound Eyes Sleep Powder. However, Vivillon sits in a somewhat awkward Speed tier, leaving it outsped by Yveltal, Mega Rayquaza, Marshadow, Arceus, and Ultra Necrozma before it starts boosting with Quiver Dance. Furthermore, Vivillon is incredibly frail and doesn't have the best defensive typing, so it's 4x weak to the ubiquitous Stealth Rock, doesn't have room in its moveset for a reliable recovery option like Roost, and is picked off fairly easy by priority users such as Arceus, Mega Rayquaza, and Marshadow. This makes it hard for Vivillon to set up, since it needs team support and can only set up against slower foes. Nevertheless, Vivillon is the very definition of a Pokemon in the Anything Goes metagame that can spiral out of control easily if not immediately assessed as a threat, and although it is somewhat niche, it can be a dangerous threat if left unchecked.