The Loa

The Loa are a naturally occurring divine phenomenon. For every beast there will always be a finite number of available souls. These beast souls are not as potent as the ones present in sentient beings. When a beast dies its soul is returned to the collective, and when a beast is born one of the souls from the collective with enter the new animal. If a beast is born without access to a soul a monster is born in its place.
The collection of souls forms the divine being known as a Loa. There is one for every beast in existence. Loa possess power on par with a lesser demigod and intellect on par with an average human. Most Loa cannot interact with the physical world and instead live their lives as powerful ghost like beings. On occasion a Loa with an important task in mind will fragment of a small portion of itself to bond with and seek the aid of a mortal with agency on the material plane. These fragments or "Lesser Loa" will guide their host to mutual benefit until the host perishes and the soul return to the collective.
When the entirety of a Loa's representative animal goes extinct the Loa dissolves into itself the loose souls spreading into oblivion. Some will birth monster, others will mistakenly merge into other Loa, and other will wander the realm as ghosts of their past. In the case of extinction by evolution however the Loa spirits simply transitions its identity towards the new life form.