Certain monsters, if you push them too much, push them too hard, they will get angry, very angry. and with that, it berserk would probably be some form of self buff that makes any ordinary moose “or boar” significantly more dangerous and alot less likely to flee.
Berserk should probably increase movement speed and attack speed, although that would make Zombie Hulks even more deadly if they could go berserk.
I would say the berserk trigger should be base on # of hits, prompting survivors to update their weaponry to finish the job before it happens.
Edit: it won’t be instant transformation, but there would be a moving text of monster is starting to get mad, etc.