Zombie Skeleton:
Shoots twice as fast as a skeleton and has more health than a zombie. Not too difficult 1v1, but watch out for hordes and spawners. Drops same things as skeleton.
Corrupted Creeper:
Like creepers they explode. Just much bigger. They also leave behind a slime friend who will spread the mutating biome much quicker. Run twice as fast as a normal creeper. Drops gunpowder.
Diseased Spider:
Has slightly more health than a normal spider. Venom causes nausea.
Mutant Jockey:
Zombie skeleton riding a diseased spider. Where is your god now?
Rage:
Enderman that spawns fire around it when it is attacked. It will then spawn a ring of fire around it every time it teleports.