[@LeamonZest89] Well, rather than thinking of their adaptations as an "evolution" like Pokemon, think of it more like how working out a muscle damages it, but when that muscle heals it becomes stronger. Maybe a Damiyo Hermitaur just naturally grows one claw larger than the other as it ages, like a fiddler crab, but because Hunters are used to succeeding when they hunt them, or because most of the ones that get hunted are younger, that size difference isn't always readily apparent. Or maybe that claw is one that got its shell broken a few times by Hunters, but the Damiyo survived and got away, and its claw healed up stronger. Maybe the only reason Deviants don't have individualized features--for instance, a Damiyo with a Duramboros skull instead of a Monoblos' as its shell, because Hunters broke that one--is due to game-limitations and thus not all Deviant Hermitaurs are Stonefist Hermitaurs. Plus it's pretty clear some of them are just really old--Silverwind Nargacuga, for instance, just seems like its fur has gone white with age like a real-life silverback gorilla or something.

G-rank Quest monsters, whether they're the normal G1-G3 monsters or the special Everwood quests, are considered stronger simply because they or their environments are much tougher than normal. Same reason not every human being is Usain Bolt, Michael Phelps, or Bruce Lee--some monsters are just born with talents others don't have, or over the course of their lives they develop differently. Like the Deviants, they've probably survived a few Hunts, and if they share their territories with things like Deviljho then the weak ones will all either be run out of the area or culled from the genepool.

Your understanding of the Apex condition is pretty much on-point--Monsters and Humans react to it differently, probably for the same reason real life diseases don't always affect two separate species in the same way. But the fact that Humans, if they deal enough damage while under its affects, also get a Boost and not just a debuff, implies to me that the Apex State is just that same concept working for the Monsters. In their case it's permanent, and what's more they often add the Frenzy Virus's effects to their own attacks. That probably means that the Frenzy Virus is still actually inside them--like a Human who's carrying an STD but isn't showing any symptoms of it at the moment. So if the Frenzy is still circulating in their blood stream, and we assume that the virus can multiply itself like normal virus cells or bacterial infections, then their Apex state never wears off because they've become aggressive enough to overcome it like Humans, but their immune systems either aren't strong enough to purge it or don't react to it the same way a Human's does. According to the Wiki, monsters that are known to be able to be affected by the Apex State are Deviljho, Diablos, Gravios, Rajang, Seregios, Tidal Najarala, Tigrex and Zinogre, so your hypothesis that only stronger monsters can survive the Berserk nature or the Healing Debuff long enough to go Apex is probably right too.

Personally the thrill of the games to me is overcoming each and every challenge, even if I do ragequit sometimes if a particularly difficult Quest gives me too much trouble. So the idea of including all this in the RP, along with things like the Burst Species or other rare variants, appeals to me just like fighting them in the games.