Ferocious beasts are beasts who have immense power, but have yet to have breakthrough to the Golden Core Stage! Demon Beasts are evolved Ferocious beasts who go through a transformation after breaking through to the Golden Core Stage!
Ferocious and Demon Beasts both have elements, as well as a core, which keeps some of their elemental powers and cultivation after they die. The Core is found in the head of most of the beasts, making it an even more obvious weak spot.
There are also sub-classes amongst the beasts, which establishes an easy-to-follow pecking order amongst them.