First, there's these sprites for the Ultra Beasts:

Addhesive pretty much just pulled a Hoopa-Unbound in here. Yellow poké kind of reminds me of Mewtwo. And it seems like people were right regarding that old theory about Necrozma's true form.
EDIT 1: According to one of SM's dataminers, that yellow thing is named Zeraora, and it's a mythical pokémon, pure electic type, "fast and offensive" (which is means it's probably a glass cannon). The rest of the new Ultra Beasts are named Poipole, Naganadel, Stakataka, and Blacephalon;
not sure to which name belongs to each UB.
EDIT 2: Addhesive is named Poipole, it evolves into Naganadel via level up after learning Dragon Pulse, becoming Poison/Dragon.
And then, there's the battle themes from the bad guys:
https://clyp.it/xs3g0vyx (???)
https://clyp.it/mu1uxpu0 (Lysandre)
https://clyp.it/1wq43mlr (Ghetsis)
https://clyp.it/bumhzoym (Cyrus)
https://clyp.it/yac4pn5m (Archie and Maxie)
https://clyp.it/cwyrfwdt (Giovanni)
https://clyp.it/o24xnkja (RR grunts)