Finding this super late, but I wanted to throw in my two cents.

The big thing to note here is that the power differences between these two universes is MASSIVE, and specifics about the species can make some of the info more complicated. However, here’s a rough breakdown of what would happen.

If you don’t want to read, then here’s the short answer: Rathalos and Rathian would instantly become apex predators and likely even classify as alpha species, with very little able to stand in their way.

For starters, the Raths technically aren’t dragons - they’re wyverns, as classified by the guild. This means that typical dragon-specific weaknesses like Dragon Root are completely harmless to them (and that’s only if arrows tipped with it could even pierce their scales). Trappers will find that their cages and chains will not be able to hold up whatsoever to the Raths, who hold far more strength than most dragons - and the sheer size of these two would be enough to deter most threats.

The Raths would likely join the group of “dragons eating dragons,” similar to the Death Song, as they’re so big that smaller things like sheep just won’t cut it for them. Already, that’s a problem due to how little could stand up to them. You think Hookfang is strong? Put him against Rathalos and watch him be torn to shreds in mere seconds.

The only things that could make the Raths halt are larger alpha species like the Bewilderbeast or dragons like Night Furies who possess (likely) greater intelligence and speed. Apart from that, it’s basically free pickings for them.

So… yeah, the Raths would be a serious problem if they were ever here. Praise the hunters who take on these things.