This is something that I hope will become a regular occurrence (time permitting), I'm going to roll up a totally random creature for use with Mutant Future/Labyrinth Lord. I'm using the Random Esoteric Creature Generator from Goodman Games to generate the creature. (I highly recommend this book if you want to throw something really different at your players.) The first entry in this series is: The Porcupocalypse!!!!
No. Enc.: 1 (thankfully)
Movement: 120' (40')
Armor Class: -3
Hit Dice: 17 (I kicked off this process by rolling a d20 to determine base HD)
Attacks: 1 (Spines)
Hoard Class: None
The dreaded Porcupocalypse is only talked about in hushed whispers across the wastelands. It is said to be a massive, 30' tall porcupine that walks on two legs. Its spines are said to be like massive spears that can skewer a man with but a swipe of its powerful arms. It supposedly feeds on the fear is causes. As it strides across the wastes, blood rains down on the ground from its body; fouling the air and marking its path. However, few ever seem to get a good look at the creature as its form is blurred as if it's not totally in sync with this reality. That may explain how it's able to vanish into the wastes after leaving a settlement trembling in fear or worse.
The Porcupocalypse is a terrifying foe with a dizzying array of mutations/abilities. First, for its size, it's quick. This gives the creature a +3 bonus to attack rolls. The blood that drips from the creature is corrosive, so with each successful hit, the target's AC is worsened by 1 point. Plus it deals an additional 1d6 points of damage on a critical hit. If that wasn't enough whenever a character is hit by an attack, he must make a save vs poison or contract a blood disease that will kill him in 2d10 days unless some cure is found (so far none has).
It's not fully part of this world, so the first attack against the creature always misses, but after that it fully enters the world and can be attacked normally. However, attacking the creature isn't a good idea. When it's hit, it splits into two versions of itself, each version with half the total hit point total (round down). Each time the creature is struck it splits.
As if all this isn't scary enough, the Porcupocalypse can forgo attacking in a round to focus its malice on a foe. That foe must make a save vs. stun or run away in fear for 1d4 rounds. The creature only takes half damage from acid.
The Porcupocalypse is cunning. It typically starts combat with a charge attack if there is not already a foe adjacent to it. This charge attack deals double damage if it hits (in addition to all the other horrible effects of a porcupocalypse attack). If it has its choice of targets, it will go after those that appear to be less armored in the group.
Mutations: aberrant form (xenomorphism), gigantism, special.
Wow. That one was a doozy. I was surprised that the rolls made sense. It actually disappointed me a little.