Beast cores.
Animals do not possess one but as soon as they get to manipulate mana of their own will, they will develop beast cores though that is usually happening in rare cases. Once a beast core of a beast is broken, they will either revert to their harmless form if they came from a normal animal form or they will die if they are born from in the very beginning with a beast core.
They are highly sought by magicians and relic collectors along with some alchemists due to their magic energy stored inside that can be used for different purposes. That is why beast cores are always in demand to almost everyone.