Effect of Belief

The universe is malleable to will. This effect has an enormous impact on everything from several races, religions, the use of magic, many diseases or other conditions, and even the nature of the goddes themselves. However, it is poorly understood by the current inhabitants of the realm, save a few obscure scholars beginning to hint at the edges of this Law.
On a small scale, the effect of belief does not have much influence, and other forces will have more sway. This is similar to how the attraction of water to the sides of a glass can operate when the glass is set on a table, but if it is tipped out then gravity and other forces act much more strongly to bring the water out of the glass. In small contexts, the influence of this Law is usually due to being part of a larger, cumulative effect.
When a large amount of sentient beings hold a strong belief, it begins to have an effect on the world, particularly if this belief is maintained for so many generations that the origin is forgotten. The more uniform the belief, and the stronger the faith of the people, the greater effect this Law has. It applies most easily to the natures of things that are already somewhat malleable, like the nature of the Lesser Goddes, and the way magic flows for a community.

Origin of the Beastkin

In the earliest days of humanity, families became a tribe, and began to develop a sense of community. They roamed and hunted, creating tools and finding shelter, learning about the best way to catch an animal, or which plants could be eaten or used for medicine. There were rules for how each person acted in their role for the betterment of everyone. They also resonated with the natural world, and shared stories about which things were dangerous or useful, or the reasons for natural phenomena.
Many of them paid attention to animals in particular, and often developed traditions which bonded the whole tribe into a shared identity, that the spirit of this animal watched over them. They created rituals full of song and dance, and sometimes sacrifices or feasting, where the hunters became the hunted in the sacred space, melding their spirits with the creatures who must die for the tribe to live. For thousands and thousands of years, these fundamental beliefs changed their humanoid spirits, eventually causing them to actually resemble their sacred animals. First it would be merely through these ritual plays, but over time their infants began to possess qualities of their spirit animal, often regarded as a sacred gift. Perhaps it was sharper senses, or physical traits like teeth or fur.
As time went on, hunters became farmers, caves became towns along a river, and the origin of their traits was lost to the mists of time. None of the Beastkin societies speak of those early days, when their skin was bare just like a human's, but from an Outsider's perspective, the lineage is still apparent. The Beastkin are often able to produce children with a Beastkin of another kind, as well as with a human partner. There is also a shared cultural thread in the underlying assumptions about the world that remained as their individual societies dispersed and developed their own unique myths and practices.
It is also not a permanent change, though the effects take far longer than a lifetime to adjust. One society, the Sarffi, hold their beliefs more loosely than other Beastkin, and this has drastic effects on their people. The variations in their physiology are directly related to their beliefs, with more direct competition between groups of Sarffi rather than a sense of camaraderie, even among their own families. They have also maintained division between the less serpentine and those with more serpentine traits in a way that their society almost unconsciously prefers to keep some folk apart so that their infrastructure relies on the 'sub-par' members. This has resulted in a difficult gestation, as the influence is sometimes weaker than needed for proper development of the traits that are more distant from the base human form, often resulting in premature death.
The process also works in reverse over time, in the distant future when many people begin to lose their belief in magic in general. Many of the races lose their Beastly features, beginning with the Sarffi. Some rare Beastkin maintain themselves, like the Naga, who hold onto their beliefs with religious fervor regardless of the advances of technology.

Type

Metaphysical, Astral