Godzilla and Kong species used to be enemies, how the two will fit into Legendary's canon

If monarch is indeed involved with this film, It's likely that they witnessed Kong and Godzilla fighting for dominance over the other. Kong's species could have been wiped out by the Godzilla species. Making them natural enemies.

At this point, who really knows? 

The problem with having Kong and Godzilla existing in the same period is. . . logically speaking not possible. I mean we see and even treat Godzilla as a Dinosaur, meaning he would of existed long before Kong Species could of even existed. I get this is completely fiction but I can't see Kong's species existing in a period the same as Godzilla's species. Now clearly they'll have some connection in some form of way. Maybe when the reign of the Godzilla Species began to plummet and mammals started to take over reptiles, the Kong species became the new rulers of the new world until their fate meets something equal as devestating to what the Godzilla Species experienced. And it's not to say Mammals didn't exist in the same era as Dinosaurs, they did exist, so maybe within the genetic code of Kong's DNA, it could see Godzilla as a threat of some kind. But as Huge-Ben said, at this point no one knows how it's going to turn out. We just got to wait and see. Who know, Skull Island may give clues, and maybe even more detail in Godzilla 2, but we won't know everything until 2020

If we base it off of the 1962 film King Kong vs. Godzilla, the scientist said that it was a battle 65 million years in the making. In other words, both Kong and Godzilla existed during the same era.

From what we do know, Kong's size will be upped in order for these two to fight in a serious manner.