You are playing with the "Allow monsters hostile to each other" option turned off. I added a check so that they will use spells in the monster arena.
BTW, who wins:
 Serpent vs Unicorn? Serpent.
 Serpent vs Babble? Babble.
 Serpent vs GWoP? Serpent (Not even remotely close)
 Serpent vs Talos? 50/50.
 Serpent, Unicorn, Nodens and Talos? Talos every time
 Serpent vs Zombified Serpent? Zombie, of course!
Still, monster summoning is turned off so its not truly fair ...