[@Zeroth] I get what you are saying about the Deviants but the fact is that they adapted to surviving an encounter with a hunter, and then somehow evolved new features and things like that. Like how does a Crab surviving and encounter somehow make it double in size and be a mix of the two colours of its subspecies. Also about the G rank monsters (assuming you're referencing the lvl140 guild quests) I never liked them, and it sorta doesn't make sense how they are so much more powerful. I put that one up to adding replay ability to the game, tho I will admit they get a few additions to their attacks at the higher levels (120-140) but they never got any new attacks that I was aware of. Now addressing Apex's they are monsters that defeat the virus, and in turn they get crazy strength boots (that I did find annoying. Dam Drive stone...) but this is explainable from the in game information, albeit with some things that are left unexplained. Now when you get the virus as a hunter you cure yourself by attacking things and when you don't cure yourself you get a nasty debuff. But when a monster cures themselves of the virus by attacking other monsters that's how they become apex, but when they fail to cure themselves they die. That's why I assume that only the strong monsters like Rajang, Delijho and what not can become apex. Because the other monsters are not strong enough to survive going berserk and attacking everything.

That whole virus bit is just a theory of mine but I feel it has fairly strong points. Tho I'm still not 100% on why the monsters stay Apex permanently perhaps it is again another discrepancy between how humans and monsters interact with the virus. Considering humans do get severely harmed by it unless they stand in a pool of it while effected by the debuff.