This is a very neat mod that adds a whole new dynamic to mobs in Minecraft. It allows for rare but random spawns of mutated mobs. To spawn one manually this mod also adds a new potion, Chemical X, which when thrown at a mob will either mutate or kill it.
The mutations are both physical and in abilities. Each new mutant is now significantly stronger and has new, more powerful attacks. They could be considered boss mobs now and would probably work exceptionally well if implemented in an adventure mode game.
One neat feature is you can hatch mutant creepers that can become your minions to aid in your attacks. They function just as wolves or cats do and you feed them TNT for strength. But be warned their explosions, while meant to save you, will harm you if you are too close, although they won’t attack you like normal creepers.