Mutations

The magic of Kobos is wild, tainted, and unstable. There are strange energies floating around that are rarely understood by any but the most learned scholars. This magic can do harm, or even simply make change.
Arcanists often develop these mutations when attempting magic beyond their reach. When attempting a spell above their level, they roll their spellcasting check as normal. If they roll a natural 1 or fall short of the DC by more than 10, they roll on the Magical Mutations Table.
Other reasons to roll on this table are exposure to high level Bestow Curse spells (7th or higher), some traumatically violent magical events (the sort that end cities) and exposure to high level hostile magic in Wild Magic Areas 
When you may be exposed to such magic, roll a DC 15 Charisma Saving Throw. If you fail, you immediately display the mutation rolled on the table.

Other ways to earn mutations could be exposure to mysterious energies, particularly powerful entities from the Far Realms or the Abyss, or eating the flesh of Aberrations. Additionally, overuse of magically infused materials can trigger these changes. If you have three active potion effects on you at once and drink another potion, or subsist for more than a week entirely on magically generated food, you must make a DC 16 Constitution Save. Unlike the Magical Mutations, these Biological Mutations are generally not instantaneous. They take 3d4 days from the moment of exposure. 
Taint Can also have this effect.

Reception

Typically, those with mutations are looked down on rather harshly by the Maecodian Confederation as a whole. This is beyond just the dangers posed by some mutations, but also due to their origins. Enough mutations are a result of trafficking with demons and hags or dabbling in cursed areas that anyone with noticeable mutations is inherently suspect. In places like Cambreath, this is usually simply a wide berth and perhaps a little extra scrutiny by the guard. In the Northrealm and Wupan, things are likely to get more violent as cultists are more of an issue in such places. In Eldermeare, the motivation for the disgust goes beyond prejudice. The majority of mutants are actually simply the poorest citizens, who rely on the Murlynd Charitable Society to feed them with conjured food. Such people are often unable to reliably afford even the one meal a week required to prevent their change. This reliance on the free services of the Murlynders cuts into the food production profits of the Carcolh mines. 
Worst among those who would harm the mutants is the College of Questions, who regard the existence of mutants as a blight to be cut out like a cancer. Ironically, the habit of the Witchfinders to array themselves against the machinations of hags and demons, and their tendency to explore tainted and forbidden places, means they themselves have a high rate of mutants over the general population. Mutant witchfinders are rarely treated with any more respect or mercy by their fellows than would a mutant beggar.