We don't know what new monsters will actually be included in the game come November, but there's still time to hear about more new content between now and then.
Nintendo has released a new trailer for Pokemon Ultra Sun and Ultra Moon, the upcoming alternate versions to last year's installments. In these new versions of the game, "a story unfolds on a grand scale, with the Legendary Pokemon Solgaleo, Lunala, and Necrozma at its center". The games are a somewhat sequel, somewhat remix of the hugely popular Sun and Moon releases for the Nintendo 3DS, though we still don't quite have all of the details, or really any major details.
The trailer also teases that "new secrets of the Alola region will be revealed", which will presumably relate to the Ultra Beast Necrozma.
Solgaleo is the Psychic/Steel creature acquired in Pokemon Sun and Lunala is the Psychic/Ghost type from Pokemon Moon.
Also discovered in the story is a new Z-Power Ring which will be acquired in-game and can activate to use a wider variety of Z-Moves.
The new trailer below for Pokemon Ultra Sun and Moon promises that new secrets of the Alola region will be revealed. "And the games also feature new Z-Moves for certain Pokemon!" Your guess is as good as ours.
Rockruff has also been given a new evolution form - Lycanroc Dusk Form.
Pokemon Ultra Sun and Moon's Dusk Form Lycanroc. The moves that Dusk Form Lycanroc can learn combines characterisitics from both the original Midday and Midnight Forms in that it can learn both Accelerock and Counter. It reiterates that only special Rockruff will be able to evolve into Dusk Form Lycanroc; everyone who picks up a copy of the game before January 10 will receive one such Rockruff as a free DLC gift. However there is still time for an announcement.