One idea that has been around for a while is zombies are the result of an infection. It may be bacterial, viruses or something else, but it is an extremely communicable disease. Though zombies usually don't mutate, that doesn't mean the disease can't. So here is a few ideas on how to make mutant zombies for d20/OSR games.
First off, to make a zombie that can spread its infection, the creator has to include Contagion, Cause Disease or something similar. Though the original zombie may be under the creator's control, the infected are not (at least until they are rebuked or commanded). The process takes a ritual and costs money and/or xp. The disease is selected by the caster and the quicker the kill, the more expensive the result. The zombie may gain a bite attack but if not, it may be able to spread the disease through contact (which does include consumption of its flesh). The disease is also important in what it can affect. The more limited it is, the more expensive the result. That means a plague that only turns humans into zombies is much more expensive than a disease that affects all the "animal" creature types (animals, humanoids, giants, vermin, dragons, magical beasts, monstrous humanoids, etc.). Obviously the GM has final say on how expensive the ritual is and how many zombies it can create at one time.
That was the easy part. Making a mutant zombie is a bit more complex. I was considering Polymorph Other but that limits the idea to arcane casters. Then I had the thought of using Monster Summoning/Summon Monster and allowing the zombie to take up to 3 traits of the summoned creature. Which does work for d20, but not for the OSR divine casters. After staring at the cleric spell list for AD&D 1e for 10 minutes, I finally decided on Plane Shift. It acts like the radiation that I mentioned in Wisdom from the Wastlands 7 ( http://www.rpgnow.com/product/100439/Wisdom-from-the-Wastelands-Issue-%237%3A-Planar-Creatures-%26-Concepts?term=planar+creatures ). I understand that Plane Shift does not work that way, but Gate is simply far to high a level to allow most players to make their own mutant zombies. Heck, Plane Shift is 5th and Monster Summoning I is 3rd, so arcane casters have a leg up there. But Monster Summoning is also limited to what creatures it can bring (at least using the charts in the DMG or the 2e PH)- this isn't an issue for Plane Shift. Of course the GM or player could come up with divine versions of Monster Summoning to make the problem go away. In any case, the Monster Summoning/Plane Shift spell just brings the essence of the creature, not its body.
The traits that I suggest for such mutant zombies are armor class, standard attacks, special attacks and special defenses. So a zombie that is influenced by the spiritual energy of a dracolisk (MM II p. 55), it can get an armor class of 3, a bite attack that does 3d4 points of damage, claw attacks that do d6/d6 points of damage, poor flight at a speed of 15", the ability to turn creatures to stone with its gaze or line of acid 3" long (that is 30 yards in 1e). And of course, it gains something of the dracolisk's appearance- a reptilian head, scales and possibly wings. Now that is a mutant zombie. I am not going to even try to guess how expensive it would cost- which should be very high considering the mutant can make more of itself with that bite or claws (or its usualy slam if it has neither).
The downside of all this work is a zombie is a zombie. For a humanoid, it has 2 hit dice and is easy to turn and destroy. The caster would have to pump yet more power (and cash) into it to make it more durable. But the results of a pumped up zombie that can make more of itself can be just what the god of the undead ordered.
Flesh golems and similar creatures may be likewise enhanced by the use of Plane Shift or Monster Summoning. The result is more complex than grafting and may include traits that are otherwise impossible for the golem's creator to provide. In 3e/Pathfinder, you might consider the effects of these spells in synergy with animated objects. The result can be quite interesting, if extremely bizarre (the picture is of demons, but could very well be mutant golems or zombies).
