A while back, I picked up a copy of Mutational Evolution, a Mutant Future supplement from Skirmisher Publishing. I recommend this book for some neat concepts that can be used by most post-apocalyptic referees. I swiped one of the primary ideas from the book when I revised the Wilderness Encounters rules for Tempora Mutantur:
Evolution in Tempora Mutantur is occurring at an accelerated rate. Once the PCs encounters a particular group of creatures, the referee is recommended to modify the encounter entry – adding a new mutation using the tables from the Mutations supplement. The referee will need to decide whether to add the newly-rolled mutation or replace an existing perk or flaw.
The next time the same encounter occurs, the creatures will not be identical to those which were previously met. This improves game play by keeping players on their toes – they will never know exactly what a given encounter might hold.
I’ve actually been trying to follow this guideline in the campaign. For example, one of the Surface Expedition Teams first encountered cerebral rats when looking for the Eden Prairie Vault Complex.
Next, during their search of the abandoned missile silo from the Underground Con session, a different party discovered hairless rat-like creatures with deformed faces dwelling within the vents. I had taken the original cerebral rats and rolled “bizarre appearance” as an additional mutation.
In preparation for the next time a group encounters these pests, I have rolled once again for a mutation. I got another flaw, “missing limbs”. I’m picturing that this means that the creatures waddle around on their hind legs like a duck, with their front limbs completely atrophied away.
So here the current version:
Cerebral rats are small, hairless creatures with insect-like eyes (arrayed vertically) and only two limbs. Swarms of these pests are feared for their psychic attacks.
Rat, Cerebral (#Enc 3d4): HD 1d4, AC 7, MV 9″, SV +0, brain bite (psi Atk, 1d4, save vs. Will to negate), or bite (melee Atk +0, 1d4), 50 XP. Bizarre appearance, missing limbs (-2 to Might).
Not every encounter has gotten this treatment, but I have “evolved” giant ants and the hangman tree as well.