I think you'd need different rules depending upon the scenario as the two are going to be bound together really.
If you have predatory beasts vs. sentients (eg Ghrishak vs. Fubarnii) then they should play just like any other faction with Kopas, Sempas & Jentas.
If you have hunters vs. herd beasties (eg Devanu vs. enuk herd) then the beasts should be trying to get from one side of the board to the other or start in the centre of the board and attempt to leave it. Apart from that they should work like any other faction with Kopas, Sempas & Jentas. If you have herders protecting the herd then they should lead the force but obviously they have the added benefit of offering protection.
If you have sentients vs. a big beastie then one should be ambushing the other. Both sides working like normal except that the beastie loses if dead, obviously
If you have hunters attacking a big beastie in their nest, perhaps they have young to protect, then the beast should have to attack or chase off any models that get too close but must always return to the nest.
By tying the beasts into the scenario I think you'll get a more "realistic" game as beasts motivations are more basic than a sentient beast, even if the ultimate aim is the same; eat,
MAKE BABIES, sleep, keep young safe.
HN
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lol
HN watching to much Game of Thrones I reckon.