Flying Wyverns: Similar to the dragon these creatures evolved in a normal way but had a breakthrough during the predatory times. These lizards evolved shells that could protect from the more dangerous predators such as ancient fanged wyverns and brute wyverns. The shells were still not enough and over time grew bigger and evolved wings. But while this happened above ground creatures even older than Zhegrus grew wings too, but left to live in the caves were unlike the other wyverns they didn't need shells. soon after wyverns became more wide spread and took over the title of main monster.
Leviathans: These creatures started off from the wyvern family but like real life Mosasaurs took to the sea. Instead of front arms or wings, they grew fins which made them rulers of the sea along with a few other species. Some species like Lagiacrus are still land lovers meaning they could be related to real life Crocodiles.
Fanged Wyverns: These creatures also evolved from a relative of the wyverns but became the first mammal like reptiles in monster hunter history. Unlike wyverns these creatures stayed four legged becoming more adaptable to land than most other species.
Piscine Wyverns: These species are technically fish but evolved from the commonly known lung fish. These creatures were very settle until both leviathans and elder dragons took to sea. This time mean they had to grow bigger and more powerful to compete with such strong creatures, but in one of the great seas of monster hunter a severe drout went on and land changes lowered sea levels completely thus still giving time for these tricky creatures to evolve to such changes and evolved a special type of scale to keep them from drying out. Soon before the Old desert made its appearance these wyverns learned to swim in sand after flopping about to get to other water sources nearby.
Fanged Beast: More of the later evolved species, these creatures evolved to mammals after the furry breakthrough of Fanged Wyverns in a similar manner to today mammals from mammal like reptiles. There where also three classes. The Sicusimia was homed to Blangos, Blangongas, Copper Blangongas, Congas, Congalalas, Kechawacha, Emerald Congalalas, Ashen Kechawacha, Golden Rajangs, and Rajangs, while the Dorsillignei where home to Volvidons, while last of all the Sicutestumoni was homed by Lagombis and Arzuroses.
Elder Dragons: Some dragons are not technically dragons but still own up to there title. These creatures evolved from a similar fish to the celeocanth but instead of five fins they had six. They soon became the world wide famous basilisk lizard (not to be confused with the mythical or the real life basilisk lizard) having six limbs. the front limbs were only used to grab thus being a very clumsy monster although strong was still very weak. It evolved wings soon later from there pointless arms and also grew shells for protection. Also they had a good elemental advantedge like the fanged wyverns, although it is know questioned if dragons are slowly losing there elemental powers becuase of there high adaptation to land and sky making them really tough, most likely posing that Alatreon was one of the first elder dragons to live, although the Fatalis had a longer lifespan.
Brute Wyverns: This catagory was homed by the bipedal brethren of the past. These creatures were technically the dinosaurs of monster hunter while being the largest and strongest out of all monsters. These wyverns didn't technically need elemental strength to help them upon killing or defending becuase they homed the strongest brute strength back then. even though brute wyverns had a small fork in the road where some kept there front legs and used them as arms while others stuck with biting and charging damage and grew smaller arms. This could posibly explain why Brachydios doesn't bite or charge like other Brute Wyverns.
Amphibians: Being similar to the widely known Piscine Wyverns these creatures are actually Amphibians meaning that there are four catagories (Technically 5 if you count the newts) , 4 of which successfully made it past the wyverns breakthrough. It is unknown if caecilians made it, but it is most likely these caecilians made it into the more known khezu by burrowing deep underground and storing body fat for winter until the ice age of monster hunter happened. The body fat stored up surely helped posibly meaning that they soon evolved to cold places and after that went up north/south. The toads and frogs how ever stayed in there natural form but also had to adapt to stuff. Zaboazagirus ancestor was similar to the rare Greenland Shark and was most likely a salamander becuase of its streamline body shape. It could be possible that its ancestor the Artic Sharq later evolved into a species of salamander still posing its shark like appearance. While the Tetsukabura evolved from a poisonous species of tusked toad. These toads later evolved a shell and slowly decreased on there poisonous ability. Soon there small tusk gradually grew larger and more usable. The Sandune Zaboazagiru evolved in a similar manner of that of the Cephalos, but this species was still a Zaboazagiru it just migrated wrongly during the great drought it had to evolve in a more terrestrial way. Also the Brown tusked Tetsukabura lived in areas that the normal Tetsukabura lived, though it used the blast element meaning it had to deal with extreme heat making its face different colors from its body.
Bird Wyverns: Evovled from the more commonly known Flying Wyverns, these creatures over time found uses for beaks and feathers and became smaller over time. Thus meaning they were definitely more agile and intelligent. This most likely explains why the Velicopreys ancestor had the ability to glide.
Carapaceons: These came from lesser monsters like crabs and scorpions, though scorpions should be in a different class, they have evolved from the more known lobster unlike real world scorpions. Becuase of the Piscine Wyverns they had to get larger and stronger and over time stuck with their ability to thrive in groups and became more defensive than there weaker counterparts. Since they were large and had to protect their vital weak spot, the shell did no good for the full grown adults so they needed something else. They now scavenge of of the skeletons of stronger, and larger monsters since they outgrew their tiny shells.
Will add more soon. Sorry if grammar and crud is bad. I just wanted to explain what I think happened and if anything needs to be added to a certain catagory or removed just say so. :)
Also forgot to mention the new sub names are not the official names, I just nicknamed them that.