In Arrival of Beasts the beasts, beside the bossbeasts, are the biggest enemy of you. To get more information about them you can look through the beasts listed up in the following enumeration of the beasts:
Much more beasts coming soon ...
The beasts always get spawned by the spawner buildings and mainly try to kill your protectbase. Furthermore the ranged beasts will try to destroy your towers on the way to the protectbase, if the range allows it. You can always click on a beast (in-game) to see all details about it.
Some beasts will be resistant to some module effects.
Beasts can always be hit by the moduleffects, read more about it on the modul page. Try to kill them with your towers and / or skills. To fight effective against the beasts it´s important to know their attributes and properties.