Slithering through dreams (Snake Isekai ?)

Bluesky Snakes - A rarer, transitional evolution of a snake. Tends to evolve into one of other options before becoming sexually mature (as such their eggs tend to fetch a high price to collectors), or die due to being snatched when lying around on treetops/mountaintops. 

Those that survive usually have good dodging/sensing skills and become Elusive Lightsnakes, capable of merging with the sunlight itself, coiling up and down on the sky, swimming through god rays inside forests, sustaining on its mana. 

Others become Skysnakes, slithering above clouds, eventually maybe even pure air-water mana spirits (known example of Contracted spirit of Desert Kingdom kings, which allowed them to control the water, and so, the people, until defeated by ████).

Finally, they are also the only known variant that can evolve into the elusive Rainbow Snake - follow it if you want to reach the Forbidden Gardens. The Guild would like to remind the reader that while everyone thinks they want to reach forbidden gardens, You DO NOT want to reach the Forbidden Gardens. Only regret awaits those that reach forbidden gardens, even if all the stories are true.

Whiskered Moonfish - A type of nocturnal fish, which gathers mana from moonlight. All adults who reach sentience must declare allegiance to one of the moon clans, of the moon seas. It gives them the ability to travel between all water basins that are currently reflecting their chosen part of the satellite, as well as being strengthened when under their light.

According to the information from our searaces branches, there are also variants that align themselves with the darkside ocean. They are however rarely spotted (or, those who spot them rarely come back), as they usually live in deep waters. All of them are at least B rank, as such only a handful of specimens were ever studied, all by now disintegrated or used as ingredients. Contact “The Adventurers Guild Guide to Skills and Monsters (searaces edition)” for more information (as the author cannot hear, let alone understand the voice recorded in Vibrationpearls, he can only suggest you ask a friendly waterkin for help)

Mana core - like how most known species have hearts, yet all of them different in size and shape, so is mana core.

In physical species, an organ usually situated next to heart or lungs, pumping produced mana into blood and transporting it throughout their body. In races whose bodies are more mana-conductive, it seems to be just a crystal somewhere near the center of their body (usually spherical, or a small sculpture-like rendition of the owner at the moment of formation).

Creatures that lack the mental abilities to control mana with any finese, yet have been exposed to its energies for a long time, might form the core just next to the organs that consume it (i.e. close to teeth/claws/shell)

Evolution - all races can experience evolution. Sometimes it’s a forced one, in which failure means death, (e.g. most egg born races) sometimes it’s an expected result of normal life experiences, (e.g. humans evolution from adolescents into adults (thought there is the famous example of the Duke of Erson Valley, who somehow stayed an adolescent until his late 60, and only advanced into adult as he saw his wife cheating on him with the gardener, on top of the flowerbed he personally raised to give her as a surprise present for their anniversary). 

As most organisms tend to develop similar skills and wants as their reaction to given environment, there tends to be a typical path which is taken by ~50% of intelligent races (often trying hard to obtain more powerful variants) and ~ 80% of other races. As the creature gets older, and continues to specialise, it becomes harder and harder to evolve into something much different, that is picking two magical evolutions in a row, even if you obtain necessary skills, makes it less likely you will obtain a physical related evolution later on