Hello, Wikians! This is my take on the evolution of Monster Hunter Creatures. Yes, it does have inspiration from some other sources (including users from this wiki).
A major contributor (especially on the art style in the later picture) is Cottonmouth255.
I believe that, like on Earth, the first life was Bacteria. The course of evolution, of course, went quite like Earth as well, until at last Fish were born. This is when the first split occurs.
Note that I use the in-game names to refer to families, and not the monsters as seen in game unless specified.
Fish Family Tree
The most ancient of all things, the Fish family that did not evolve remained largely unchanged, except for minor changes that occured to suit them better to the areas the frequented; and these are not documented. However, the fish soon split, isolating the Fish family and making two more: Piscine Wyverns and Insects.
Piscine Wyverns evolved from Fish quite early; they were the first to even go near land, and thus their limbs developed much better and faster. The fins stretched after they came closer to the surface; it became common for the various piscines to jump as a form of transportation, and due to increased deep-sea and increasing upper-sea level predation, the Fish were gradually pushed even closer to the shore--resulting in the Piscine Wyverns.
Piscine Wyverns' main high point is their large limbs and able bodies; not quite as slow, bulky, or powerful as Leviathans, but also much faster, more agile, and typically more streamlined and able. They are less apt to spend a lot of time on the land (in fact some "Land Swimmers" do not even have legs) but have frog-like specifications when they do.
Yes, Epioth gets its own header. Why? Because it just simply doesn't fit anywhere else. I predict that a few of the Piscine Wyverns were quite omnivorous, and a few that weren't as good fighters as escapers began avoiding conflict (and swift death because) by eating vegetation in increasing levels; eventually, it became their main staple. Thus the Aquatic Herbivore class came to be.
What's the Difference?
But Cead! What's the difference in Leviathans and Piscine Wyverns, then? For one, age. Piscine Wyverns are at least several hundred years behind even the shadows of modern leviathans. Piscine Wyverns are also known for being remarkably more fish-like, and very, very amphibious; most Leviathans are quite amphibious as well, but they tend to rely more on water preservation and storage rather than the simple stay-wet-until-you-can't method, as Plesioth uses because of its gills. Finally, some leviathans don't need water--this is because they have lungs, and not gills at all.
The first insects came about roughly the same way Earthen insects did. The first ones were the Neopterons; originally, they were many times the size they are in Monster Hunter Tri, for example, and were much stronger. Then, of course, came the smaller insects, evolving from ground-based varieties who were forced to grow smaller by predation tendencies.
After a while, the Piscine Wyverns got even more pushed to land--at least, those who were not still on the open sea (but on the upper layers); some of them began to evolve lungs, as opposed to gills, which allowed them to surface constantly and some even needed to stay out of the water. Some, like the Lagiacrus and Royal Ludroth, began going further and further inland, some, in fact, losing their gills and being able to wander farther and farther from water; thus were formed what we know as Leviathans. Agnaktor, of course, evolved directly at the volcano, and its subspecies likely formed as some sort of genetic mutation found en masse at some point, so that all creatures that had the mutation were shunned and driven away. The mutation reproduced until they found a suitable environment. Royal Ludroth came from the common ancestor of such creatures as Lagiacrus: however, this common ancestor (which became quite land-oriented, only going to water for mating and refreshment) split, leaving two subfamilies: Amphibious Leviathans and Brute Leviathans, such things as Royal Ludroth being in the former and the Lagiacrus and Agnaktor in the latter.
This, however, began the next split: Leviathans continued, but two other trees at least were created, one near the beginning of Leviathans and the other near their "modernization": Basic Land Wyverns and Deep Sea Dragons.
Deep Sea Tree
The Deep Sea Dragons consists of Ceadeus, and Goldbeard Ceadeus. This tree broke off shortly after Leviathans took to land; these creatures were the closest descendants (and indeed the closest in species age) to the ancient huge fish that had swam the deep waters before the split, and they were a bit overlarge for successful land dwelling. Because of this, they developed a few land-typical characteristics (one of which being a pair of vestigian horns that at the time were nothing more than a hard plate) before returning to water. They first returned to just lagoons, still surfacing occasionally; but about the time of the Lagiacrus common ancestor's beginning, they took to the deep sea to find it much less populated than ever before. The fish, due to the lesser competition, limited range of food, and lesser seas, had shrunk and began to evolve strange ways of species survival; some exploded on death, some flashed light at death, etc. But the Deep Sea Tree was not concerned of this. However, as a few Amphibious leviathans began to take back to water, they began to retreat to deep seas; and thus they passed out of all knowledge of any beast for thousands of years. They dwelt in deep caves, never revealing themselves, until Lynians came about.
Basic Land Wyverns
Basic Land Wyverns were an ancient common ancestor of all land creatures. Their tree splits several ways.
I'll agree with Cottonmouth; this one is definitely older than any other species as a whole (as far as Land Monsters are concerned) and came about at this time. Originally, this was a reptilian development designed for one purpose: 'kill, kill, kill. It was and is a supreme predator, being rare because of its power; it is so feared that its finding food is a rather hard endeavour, and thus the population cannot increase as well--like the Deviljho.
The first ones to be an obvious split were the Brute Wyverns. Eventually, many creatures evolved to their working design; not all Brute Wyverns came from the same exact ancestor, although they came roughly from the same sources. The first Brute Wyverns were even rougher and more powerful than the ones seen in MH3U and MH3; they stood about 15 feet tall at the top of their shoulders, and typically around twenty at the top of their heads. These creatures were all raptor-esque, having two standing legs and front arms that were not used for walking. However, their front arms (contrary to popular belief) were never useless; some, in fact, had long and strong arms usable for eating and other such important functions. Most of the time, however, their use was limited to mating, competition between members of the species, and (the ones who built nests) nest-building. Then, another split occured: Shelled Wyverns was one major change, and Hided Wyverns were the other.
This consists of Brute Wyverns like the Deviljho, which has hide and small-ish scales as opposed to large plates; most of this classification died out long ago, leaving few (like the Deviljho and the descendants of the Hided Wyverns) left.
Deviljho is another living fossil; it evolved right about now, and, due to its extreme advancement, it never evolved.
The Fanged Wyverns were the eventual descendant of this class; their soft hide was not a good advancement for long, and evolution (and instinct) told them that they needed to be faster and more agile. Their hands and arms slowly developed farther, resulting in the very first Fanged Wyverns; a creature very similar to a Brachydios--if its body was closer to the ground and its leg shape changed greatly (in addition to the many color changes, and the lack of a big green horn, etc.) These evolved on their own, and did not split again too greatly.
As far as my own story and invented monsters go, however, they did split again. The Urruduk is a great example.
These split near instantly; the main split, of course, was the Wyverns, which have strong, platelike shells, and can typically fly; the Rathalos and Diablos, for example. The other split was the Brute Wyverns like Barroth, Duramboros, and Uragaan. These split into Volcanic, Forest, and Desert species, each one evolving slowly; the Uragaan split again, forming the Modern Uragaan and Brachydios--the latter of which was not discovered until many years later. The Brachydios, due to its hard shell and able heat and cold resistance, was able to venture far (typically via land bridges) and wide, which is (of course) why you find them everywhere.
The Flying Wyverns evolved slowly, with an ancestral creature like Diablos being the first of them; Monoblos, of course, emerged directly after, and from them came creatures like the normal Rathalos and Rathian. The reason for this is that the population of these ancestral creatures reached points in which their old homes would not contain them; they had to develop flight skills to catch food more easily and faster, and this reduced the strain; for some of them came to fly far away from their old homes, where they evolved in areas with much less strain. It can be assumed that after a long time, some of those wyverns meandered back to their original homelands (example being the Rathian).
In the course of evolution, some of the ground-lying wyverns evolved stronger wings, more suitable to running and flying low; they eventually devloped strong forearms and claws, able to hold on to the ground or run on their wings for short periods. This tendency grew until they weren't flying nearly as often (compared to being almost constantly airborne) and were able-bodied on the ground; thus formed the Pseudowyverns.
Bird Wyverns, I predict, evolved from the Brute Wyverns with soft hides; they grew more obvious scales, and like on Earth, after many, many years of evolution, Feathered Bird Wyverns developed. After a while, the Qurupeco emerged and was the first Bird Wyvern.