Just finished Seabound  (I cried, I really did) and now my mind is full of theories.

I'll start with the basics:

Dragons = creation (doesn't mean, they are naturally "good")

Oni = destruction (doesn't mean, they are naturally evil)

We know that certain dragons are releated to certain elementary powers (like those in season one).  But elementary powers like water, fire, etc. have the potential for both creation and detruction, which means, that they have to come from Oni and Dragons equally.

I assume, that there were that kind elementary powers prefered by dragons and those prefered by Oni. Remembering, that the first master of Spinitzu was in his mind rather a dragon, it gets clear that the elements he passed on (in the golden weapons or in the elementary masters who fought in the wars against the snakes) were prefered by dragons, while those he neglected by him (wind and water) were those prefered by Oni.

Primarily there were: 

Earth, fire, ice, lightning -> Dragons 

Water, wind -> Oni

 From those he "liked"; the first master of Spinjitzu chose the four dragons prefered most, to support the golden Ninja with, so that he'd be able to kill an Oni (I am convinced, that the ultra evil was an Oni as well, but that's another story...)

Second thought: We know (also from season one) that dragon do have the power to travel between the worlds. So why should Oni be unable to do this as well (it seems that they have lost this ability - just look at  season ten - maybe because after the ultra evil entered Ninjago the first master of Spinjitzu shut the "way" between the two worlds, I'll have to think about that later)

Then one Oni - called Wojira - traveled between the worlds. She landed in this ice-kingdom (Lloyd called her name, when he walked through this canyon to cause an avalanche), which would explain why they knew name there. During that time the first master of Spinjitzu started creating Ninjago, with it's landscape and it's humans (elementary masters) - creatures of both dragon- and onipowers. Wojira came to destroy that world and achieved it. The first master of Spinjitzu admitted that he'd need the help of someone with the same powers (storm and wind) Wojira had, so he allowed wind and water as elementary powers in his world (what explaines, why they exist, but are not part of the tournament of elements, not present in his scrolls, etc.). Nyad killed Wojira and the first master of Spinjitzu built a new Ninjago.

Both Dragons and Oni often came into that world and left their marks on it. Therefor, the dragons from season one (in their size and power hardly comparable to the Dragons in the world they came from) are descendants of the "great dragons", while the descendants of the oni are ... the snakes, who - like the smaller dragons - have elementary powers (transforming -> like Camille; hypnothizing -> the power Neuro had; becoming invisible -> Mr. Pale; poison -> Tox).

Now both Wojira and the Devourer have in common, that they have something on their forehead, that is vital to them, since it holds their (elementary) powers. They are bouth gigantic serpents -> they are of the same species.

So maybe the Devourer was an Oni (or at least the descendant of an Oni). I mean, it's bite started Garmadon's desire for destruction; (the elementary power behind it would be poison, maybe);...

Conclusion: I proved more or less reasonably, that Ninjago is inhabited by both dragon. and Onidescendants and that elementary powers do come from them. No idea, whether this theory allready existed, but I thought about it when watching "Seabound" so, ... 

If you find anything, that doesn't fit in that theory, I'd be interested in hearing it.  

Alice🙃