The massive amount of radiation released during skydark damaged the genetic structure of every living creature on the globe, everything from the tiniest insect, plant all the way to man himself. Mutations of all sorts were born. As it is with many mutations, most of these creatures died at birth, or were miscarriage. However, enough survived to spawn entire new genetic Races such as the Stickies, Troggies, Scalies, and many, many others.
Mutations were most often than not harmful, such as physical defects. Many mutants were born with extra limbs, or twisted body parts. Facial features could be scrambled. However, quite a number of mutations were beneficial. Many mutants found that they possessed superior senses, enhanced strength, usable extra limbs, and in some rare cases, Psionic abilities.
Many Mutants that possess Psionic abilities are known as Doomies. These mutants are generally able to see the future, but usually only the bad events. There have been mutants that possessed a limited form of telepathy, as well as a limited form of mind control or hypnotism. Mutants such as these are exceedingly rare.
As mentioned above, mutants are universally hated and feared. In the vast majority of cases, it is justified. There are exceptions to every rule however. Many of the most hideous mutants can be the kindest and gentlest of people. The way a person looks doesn't always reflect on the soul of that person.
Most 'Racial' mutants share similar mutations. Stickies have suckered hands, feet. Scalies have scaled skin, and look reptilian. But it is not uncommon to see differences from one group to another. Some Scalies encountered have been incredibly hard to kill, possess no nose or hair and are animalistic. While another group have hair, noses, and are quite intelligent and live in clean neat communities.
The intelligence of mutants is not set from group to group. One group could be little more than animals, while another group of the same gene type could be highly educated, intelligent and organized. It all depends on how the parents of these mutants treated them after they were born. If they were left in the wilderness to fend for themselves, they would revert to primitive ways, where as if they were cared for, and taught, their intelligence would reflect this. In some cases, low intelligence is a factor of the mutation.
Deformities in mutant 'Races' are not uncommon either, and the birth rate is still unusually low, as there are still large numbers of still births and miscarriages due to the genetic damage. As time progresses many of these 'Races' will die out, while others will weed out the weaker members and the gene pool will stabilize.
Before the war, many species of wildlife were on the brink of extinction. Many lakes and rivers were stocked with fish, certain animals such as wolves were captured in Canada and transplanted to the national parks in the United States, and entire species were in danger as their habitat was slowly destroyed by man.
After the war, many of these species did in fact die out, but once the long winters ended, some of the species, notably the wolves, deer and so on, came back, and with a vengeance. No one had to worry about starving in the Deathlands if they know how to hunt or fish, as there is plenty of game everywhere. Many of the lakes and rivers that had to be stocked before the war are now teeming with fish. It is not uncommon to see deer walking down the streets of a ruined ville. Of course in these ruined villes the main form of life to be encountered are rats and other such vermin.
Nature has its ways. Life always finds a way.