WMG: Monster Hunter

Monster Hunter takes place in a low gravity world
Hunters seem to be capable of falling from great hights without getting hurt, animals can grow to immense sizes, especially invertebrates whose exoeskeleton can hold their weight with ease, compared to their Earth counterparts. The low gravity could also make it possible for large creatures to move under sand, and for hunters to carry absurdly large weapons—they simply feel lighter.
  • If that's the case, it could be a Lost Colony on a Death World. And somehow the descendants of the colonists haven't lost their Heavy Worlder muscles yet. Which would also explain how a Bronze Age civilization even has guns.
  • If you go to Area 9 of the Volcano in Tri, you can see a planet/moon that looks far too large to be the moon we all know. It's also a different colour (kind of purple), although the volcano might have something to do with that. So perhaps the MH series is set on a different world. It may just be a severe case of moon illusion (somehow), but the colour and size don't really seem to fit those conditions exactly.

Odd volcanic behavior
The volcanic areas present on the game seem to be strangely static, showing little change despite their active magma flows. Said magma flows work more like rivers (and waterfalls). The minerals present in these areas seem to be able to stay solid even at very high temperatures, which could explain why some areas remain unchanged, and the native creatures in these areas, by absorbing said minerals these creatures are able to resist the ridiculously high temperatures and some even dive in magma without problems.

Hunters are not human
The Hunters may look human but they are in fact some form of super human. They can carry and effectively use weapons as large if not larger then themselves, they can survive forces that would reduce a human to a smear on the ground, and they are capable of holding their breath for extreme lengths of time and draw oxygen from oxygenated water (passing through small patches of bubbles refill your breath meter)
  • It's implied that they're eating bits of the monsters that they hunt, so the hormones and whatnot from various giant critters are probably having some kind of effect...
    • Alternately, if we're going with the Lost Colony theory, Hunters were deliberately designed to ward off the megafauna that runs wild, kind of how some of the firelizards in Dragonriders of Pern were genetically augmented into dragons in order to ward off the Thread. Maybe early settlers used DNA from more primate-like monsters like the Congalala, Blangonga or Rajang in order to create the first Hunters, giving them the kind of endurance and strength you'd find in the megafauna. With regular interbreeding with ordinary humans, Hunters became more humanlike in appearance while still maintaining their nonhuman capabilities. Either that or they're the offspring of super soldiers who settled on the planet in order to get away from war. Alternately...
      • Alternately, they could be the result of interbreeding with Wyverians, as they are also quite human looking (long ears, toe talons, and odd aging habits aside), and are shown to be competent hunters (one piece of art shows a scrawny elder Wyverian standing over a monoblos holding up its horn), add this to the fact that most of the villages as well as the guild seem to be run by Wyverians...
The monsters are the end result of Gaia's Vengeance and the Hunters were created by humanity.
Think about it: when a monster takes you down, they don't eat you, they leave your body, thinking you're dead. That can only mean that the monsters are intelligent enough to know that humans are the most dangerous creature on the planet. Ignoring the gameplay mechanics and looking at the world itself, the monsters may have been the result of evolution, like humans kept killing and killing and only bigger and nastier animals were left behind. The more humans kept invading their territories, the more humans kept dying. The end result? Monsters that (allegedly) eat humans, thus explaining why they're so big: humans are fucking EVERYWHERE, so with enough humans, monsters would have tons of nutrition to grow big. Hunters were created to combat the threat.
  • That would at least explain why most monsters have evolutionary features that would be absolute overkill in the context of anything other that combat; a lot of monsters are designed specifically for combat (we all know it's due to Rule of Cool) but being able to shoot a water laser doesn't really do much apart from tons of damage.
The Ceadeus is based off of the Sumerian god Enki
The Ceadeus has traits of both a goat (The prominent horns and "beard") and a fish (living in the water and having a whale-like tail), both sacred animals of Enki. (Which were later combined to form the "Capricorn", which again the Ceadeus has an uncanny resemblance to) As well, both Enki and the spoiler: Ceadeus live in underground oceans. (Though freshwater in the case of Enki and saltwater in the case of the spoiler: Ceadeus) Also, the Akkadian version of Enki's name is "Ea", which is right there in the name: cEAdeus.
The world of Monster Hunter was, millions of years ago, invaded by the Kaiju
Just like in Pacific Rim, the Kaiju invaded the Monster Hunter world through a rift in the ocean floor. However, as explained in the movie, a pristine, earth-like world is not ideal for Kaiju colonization, so the rift was closed and the remaining Kaiju were left to fend for themselves. While some of the Kaiju's traits, more practical for indiscriminate killing than survival over a long period of time, were evolved out, such as their massive size, Multi-Armed and Dangerous traits and Bloody Murder abilities, some of their impractical traits, such as highly specific and energy-consuming weaponry and needless aggression remained. The result, of course, are the titular monsters of the game, which highly resemble the Kaiju. The Pseudo-Wyverns, who have wings that double as arms and run on all fours like cats, and the Cave Wyverns (The Gigginox and Khezu), who have similar attributes to Pseudo-Wyverns, use poisonous sacs and have prehensile tails, are descended from Otachi clones, while Brute Wyverns, who are dinosaur-like in behaviour and size and tend to have distinctive crests on their head that can be used in a charge attack, are descended from the clones of the Trespasser/Knifehead/Scunner-type Kaiju. Leviathans, who have crocodilian body structure and are the most adapted to underwater combat, come from Raiju clones. Elder Dragons are descended from the enormous, mysterious, Class-5 Kaiju.

Pink Rathian and Azure Rathalos are hermaphrodites.
The Rath species of monster is split into two parts: the green-colored female Rathian and the red-colored male Rathalos. This brings us to the two subspecies. Pink Rathians' dull pink scales make them look more like a Rathalos. They also change their fighting style to be a little more like a Rathalos, spending more time in the air and added fire to more of their attacks. Similarly, they lose the Rathian's motherly instincts (normally Rathians will come running if you pick up a wyvern egg, but Pink Rathians don't care about eggs at all). Azure Rathalos, similarly, is colored a little more like a Rathian and has more poison in his attacks, again like a Rathian. This is because these two subspecies are "in between" Rathian and Rathalos; they were born as hermaphrodites, having the traits of both the male and female of their species.

The Monster Hunter Frontier world is an Alternate Universe from the main series.
Of course, we do know that that Frontier development team is completely different from the main MH staff, but In-Universe this would certainly explain why that series has wildly different creatures from the home and handheld versions, despite sharing the same locales as the 2nd generation games. Differences in monster distribution and migratory patterns would have affected creature speciation, leading to the unique specimens found only in that game.

Monster Hunter is a prequel to Pokemon
Long before humanity domesticated the creatures that we know as Pokemon those creatures wandered the Earth, a constant threat to the developing humans. To defend themselves against these threats hunters began using parts of the monsters themselves as weapons and armour, eventually this philosophy gave way to directly using the monsters to fight other monsters.

The Cheeko Sands Elder's husband is still alive
If you talk to one of the felynes at Cheeko Sands, one of them mentions finding a message in a bottle stating "Hunter with the Sword and Shield looking for Cheeko Sands." The Elder's husband is known to wield a Sword and Shield, as evidenced by the Heroic Blade. Logically, one can assume that the message comes from the Elder's husband.

Furthermore, I have a theory as to WHY he can't find Cheeko Sands. Cheeko Sands is stated to only be found by people who lose their way. The Elder mentions that her husband had an excellent sense of direction, and can't understand what is taking him so long to get there. Therin lies the problem. Since he has an excellent sense of direction, he cannot lose his way. If he cannot lose his way, he cannot find Cheeko Sands.