General:
* A herd of beasts (if the beasts are similar in size and type)
* A pack of beasts (if the beasts are wild and hunt together)
* A menagerie of beasts (if the beasts are a collection of diverse types)
More specific:
* A pride of lions (if the beasts are lions)
* A pack of wolves (if the beasts are wolves)
* A drove of cattle (if the beasts are cattle)
* A flock of birds (if the beasts are birds)
Ultimately, the best collective noun for "beast" depends on the specific type of beast and the context in which you are using it.