At the end of the new teen wolf trailer Stiles is dragged away into the darkness but that is not before he comes face to face with some kind of monster. Thanks to this wonderful post it is believed that the creature says in archaic Japanese ““I am not a (vengeful) ghost/illusion”.
It is also important to note at this point that Stiles is wearing his pyjama bottoms so there is a high likelihood that he is actually asleep and dreaming. Yet the monster is claiming to be real despite this. Peter did communicate with Lydia in her dreams in Season 2 so it isn’t an altogether ridiculous notion.
I have decided that this creature is a Japanese demon called an Oni. Although ‘demon’ is only one translation and they are often considered more akin to a beast, ogre or a troll.
I was hoping to be able to clean up the screencaps a bit so we could get a better look at it but it’s so dark it was basically impossible. In the actual trailer you can see it moving between the bars at 1.39. Here are my best offerings of what the Oni looks like (yeah I know it’s just a freaking big blurry blob):
It’s back appears to be ridged like it has very thick skin and its face although we only have a side profile is humanoid but not human. I feel like it fits the idea of a beast/troll/ogre somewhat.
I did some research but there is actually frustratingly little on Eastern mythology to be found on the internet; websites just rehash each other and I am all like “but I want new information”. I did make a brief trip to Google Japan out of desperation but the ‘translate this page’ option lets just say the results were kind of hilarious if you can’t read kanji.
Anyway what I have found in my research is the Oni to be called anything from “the official enforcers of the underworld” and considered a part of the next stage of death for individuals who have done wrong. To being connected to what is called the Demon Gate – the cardinal direction (northeast) in the Zodiac through which it is said evil spirits pass.
What they are and what they do has changed considerably in Japanese folklore over time. Where they went from being invisible and physically insubstantial spirits that caused disasters and disease to something more solid. Oni are mostly depicted as evil, but can occasionally be the embodiment of an ambivalent natural force.
They are a kind of Yokai. While the word can be considered synonomous with ghost the Oni is usually seen as a living thing or a supernatural being which makes sense because it said something akin to “I am not a ghost” in the trailer. A subset of Yokai are Obake which are shapeshifters. The Oni is considered an Obake if it is an embodiment of nature.
However I think of the most relevance piece of information I found is the fact the Oni are also believed to have the ability to seize a person’s soul and also surround a soul with darkness. 
I believe that they are going to be exploring the idea of souls or spirits in 3b. And with the ‘door’ open it looks like maybe the souls of the dead are trying to (or have) escaped.
The monster also says in English “we will destroy all of them.” Who or what is it talking about? Could he be talking about the souls that have escaped through the door? Destroying them is different to putting them back where they came from it means they are gone for good. And all means the good souls and the bad souls.
It is unclear whether this creature is threatening Stiles or not. The last thing it says as Stiles is dragged away is probably “we are not alone”. Did it say this because something else dragged Stiles off? Something else is present in his dream?
I am actually thinking maybe it is Lydia because I have decided she has the power of dreaming walking idek at this point. Basically the Kitsune have the power to manifest in dreams and create illusions that appear as if real and there is a high chance that Stiles’ dreaming issues are caused by this trickster spirit.
But I assume Lydia is immune to these just like she is immune to everything else supernatural. As Stiles’ anchor she is the one that can draw him home – his link to the real world. I mean the first episode is called Anchors after all :3
[Tidbit: Oni in Russian and Polish translates to ‘them’ which is an amusing coincidoink.]