There are two types of beasts, heavenly beasts and demonic beasts. Heavenly beasts absorb the heavenly qi within the world, and demonic beasts absorb demonic qi within the world. They dont necessarily need to only absorb their specific qi they can just absorb neutral qi, or sword qi, or any other forms of qi except the opposite type of qi, meaning a demonic beast can't absorb heavenly qi and vise versa.
All beasts can become Heavenly or demonic beasts, but more often demonic or heavenly beasts are born from other demonic or heavenly beasts, the reason being it would take too long to absorb the necessary amount to become a demonic beast or heavenly beast, with the absorbtion speed of normal beasts, it was simply too slow. It was like a human cultivator becoming an immortal, although it happens its rare, but that Dosent mean it's impossible to speed it up, by consuming other beasts, beasts are able to speed up the process.
Heavenly beasts and demonic beasts have ranks 1 - 100, after the hundredth rank they evolve again into a, demonic or heavenly general and then there is another 100 ranks, then they become demonic or heavenly kings, then it gets different but I'll talk about that later.
How human cultivators and beasts rank
body refining - normal animals
qi gathering - normal animals
qi condensation -demonic and Heavenly beasts( low rank)
foundation stage - demonic and heavenly beasts(high rank)
golden core stage - demonic and heavenly general (low stage)
soul wandering stage - demonic and heavenly general (low to mid stage)
nacent soul stage - demonic and heavenly general ( mid to high stage)
ensoulment stage - demonic king(low stage)
nihility stage - demonic king(mid to high stage)
Also re-read cultivation levels, i added the body refining information, at least current info, later will change.